They fell for a scam. That doesn't merit the death sentence.
You know, if this scam wasn't both easily avoidable and actively calling for the imprisonment and/or death of various groups of people within the country, I might be a bit more sympathetic. I know it can be tough watching someone you know fall for it and not being able to stop it, but getting mad at internet strangers who say that MAGA followers got what they should have seen coming to them has the same vibes as going "But my uncle was a good guy before he fell for all this Nazi stuff Hitler is saying, he didn't deserve this," while overlooking that they volunteered to be a guard at one of the camps.
Nobody is forcing people at gun point to go MAGA at present,, and adopting and promoting that ideology has real impacts on other peoples' lives, which currently include arbitrary imprisonment, an inhumane death camp in El Salvador and Trump's brown shirts beating and killing people in the streets. So it might suck on a personal level, but who are you to tell someone who lost their access to abortion, medical care and/or the ability to just live a normal life if they happen to be trans or gay in the wrong state, or even someone they know being rounded up by ICE that they're bad people when they say, "Well, good thing another MAGA person died, that's one less of them fucking up my life," in a situation like this?
Again, it sucks for you that you lost your uncle, but he didn't live in a vacuum and the ideology he adopted has real, severe consequences for everyone else who has to deal with it.
With that said, I do think we need to give some serious thought to deprogramming these folks and reintegrating them with civilization in the near future. "Screw them all, let's just kill them," is hardly going ti be a viable strategy when we have to face our own period of de-Nazification in the future and could be helpful in actually getting some off the indoctrination pipeline and reducing their numbers in the present. There are millions of people who have openly embraced fascism in the US, and millions more supporting right-wing extremists in Latin America and Europe. Indefinite imprisonment or mass executions are both equally unfeasible as solutions for when we collectively pullour heads out of our asses as humans and go "Oh shit, we really fucked up with this fascism stuff, again. What do we do now?"