I consider someone anti-life that argues killing is basically okay if you think it’s okay. It’s a made up comic and not one of us really knows what the bad guy did.

The point of the comic is that many stories in comics and other media try to introduce a last-minute moral dilemma in which the good guy stopping a bad guy is made out to be as bad as the bad guy. Often it's a lazy mechanism that doesn't work from a writing standpoint because the reader/viewer has experienced the entire story solely from the protagonist's POV, empathizing with and seeing the victims of the 'bad guy', and never the bad guy's view, and suddenly asking them to empathize with the 'bad guy' at the last moment with this lazy argument doesn't actually work. That's what this comic is about.

But the reason this is likely resonating with more people right now outside of commentary on literary tropes, is that people are experiencing a world in which many governments have broken the social contract to execute justice fairly and protect their populaces, and sided with companies and wealth-holders in allowing them to harm said populace. Given that backdrop, and recent reactions by billionaires and CEOs (e.g. Flock's CEO's recent call for "compromise" in which he gets to keep surveilling people), this comic may feel for many people very personal.

I would argue that we badly need to find ways to prevent mass shooting instead of being okay with killing the shooter.

Sure, but that is dodging the question of whether you accept that killing someone may be necessary to prevent them harming others.

As for crime in general… The more civilized society is the one without the death penalty.

I agree 1000%, because a government can never be trusted to actually serve the interest of its constituents. It will always be pulled towards accumulating and protecting power. That's the nature of Authority.

Ultimately, you can't claim to believe in preserving life above all else if you refuse to stop something that is threatening life. The "pro-life" reaction to a Hitler is not to refuse to intervene in a Holocaust. You can personally choose not to take action, but you don't get to dictate to others when and what the line is between pro- and anti-life actions.