I get where you are trying to go with this and in a way you are correct. This is just not a situation this applies to. This is a premeditated attempt at first degree murder. The other people involved can be categorized in the realm of harassment, even if it is extreme it is still arguably in the realm of harassment.

Setting up a trap/ambush to kill without mercy on a specific (innocent) target, while also involving an innocent 3rd party as collateral damage is an apex of crime (IMO). The only way that he should subdue her and it be considered Morally Good is if he can make her a non-threat with I would argue a minimum of 70% certainty. If not without certainty of such a high percentage to secure her there exists chance for her to escape.

The problem in this instance is that he can not guarantee control. That is the issue; You can try to argue that a self admitted (attempted) Murder should be captured, but it falls through because we already have the context for what happens in this specific instance. Making a potentially valid argument moot in this specific context; the choice was made and in the realm reason was wrong. The thing that makes your point have merit is that in the moment -capture is what a decent person should do but fails to encapsulate that if it can not be done then killing her is the morally good thing to do because her powers can not be supressed and she is an active and willing threat every moment afterwards. (A point specific to the story/world it exists in)

Though in a real world application we do not have that future sight or such ...so sadly we can only use our best and preferably trained judgment to lead to the best outcome. (made more realistic and easier to argue with the lack of "super" powers and magic [like] abilities)
To address your point of 'don't kill unnecessarily', you have to have the potential/ability to neutralise the threat. With powers of that nature that would mean putting her in a unconscious state (or suppress her ability) AND have the ability to keep her in it.
Morality is a hard thing to exist with; Good Morality in the moment does not always equal such in the long term. Worse yet perspectives can change it in many untold ways. It would be Morally Good to save a person, but also Morally Good to kill the same person if they carried a disease that would wipe out the planet even if that is something that they have been infected with by means outside their control. A topic that can have such a situation is not an easy one and falls apart to too many specific examples when tested against general Ideas.

These comments can sometimes be top of the line! I have lulled myself into the zone of just ignoring/disregarding almost all the quality drops and have missed some funny things. Thank you heroes of the past!

Countless possible reasons aside; He may just see himself as a killer for having killed, justified or otherwise, even if the end result is a positive for the world he knows it was only for personal reasons and does not justify it otherwise. You can even argue that a person with the mindset in such a position would be more "Good" than a normal good person, because taking a life should always have a factor of guilt with it. There is a moral argument that if you can take a life of another human and feel no guilt your 'Humanity' could be questioned.

Maybe it is magic glass, I dunno, I do not really judge this one on reality. I mean if you want to point it out even further: it was a the Lotus Guild's table and it was stabbed by one of the woody boys, so I would say it goes a level beyond just stabbing a glass table - it was someone else's table...

It is I from the future, The guess was way off, the truth... well, just say it kinda fits the story a little too well...

Other comments point out that there is a good chance it is the lass from the first (maybe it was second?) chapter.

That light beam was so strong it leaped beyond the realms and rearranged dude's face for a single panel. One would need super natural powers to slap those eyes back into place and remove that frame from existence.

The story is kinda fast, but I know what you mean... I feel like I put this down over a year ago and came back to like 2 chapters. I think a big part of it is that a chapter ends and even if something gets solved in it there is just a cliff-hanger that leaves you going: wtf is that about. Then when the next one drops so long after... it is just like "yeah, ok".

Really fails to hit on any meaningful mystery type aspects for me. Ends up being more 'lack of info' than 'building suspense'.

It is good to see a Villain die, but I can not help but just sit here and say: "Finally." 

lol, thanks for the UP vote very kind of you. Though I can not really be upset about the downvote, what I mean is: in the end is that not exactly what was described? Person just kinda outed themselves...It really ends up like: Tell me you are a emotional self saboteur, without saying you are a emotional self saboteur. 
- I just wish everyone well.

Yeah, Sometimes people can just be down on their luck and need a kick in the rear to get back into the right mindset. It is nice to see that in a story, better that they did not become some no brained followers...

This is what happens when you get blackout drunk on depression while double fisting some self doubt/loathing. Basically drowned in the De-nial river but did not die. Far too many bad brain chemicals, needs to take a hit of Jet and go off the rails.

Gotta put this one into storage till there is a stock of chapters. Hits a lot of good point, in the right ways, but is WAY!!! too short!

Expecting? No...
Surprised? Also No...

You are both right and wrong, let me ruin your view of people a little more.

You ever see a dog on a leash run around this way and that, get tangled up in said leash then proceed to have a freak out? Ever see a dog do that day after day, how about a few times within a short period of time?

You ever see a person walk into a table in their own home more than once, how about stub their toes on the same piece of furniture as a near daily affair? Ever know someone that complains about something but instead of just taking some steps to fix it or putting in the effort for it to not be a problem... they leave the problem there and act like the world is against them.

If vomit did not taste awful to humans do you not think there are some that would do as dogs do? 

That is hilarious!

[Dear Diary: I have done the lords work today.]

They do have tie down straps or a system of cables and hooks to keep them locked down... but Yeah... what it would take to impale the mood should have just made them stop existing...

Worst offenders are those Helis though...

Yeah... took a few hours for that one to click for me. Was a bit of a mess before they added in the Red Exclamation Point error like icon. Though I have noticed how the new point system has really killed the amount of likes on posts...

This is too on point, well done.

I am not sure why but I will support your hopes in this.