I take it you think Brian Thompson deserves sympathy as well?
Yes. I think he was a pile of trash (esp. since my company uses his insurance company), but I will only consider violence as an absolute last resort. These types of problems can and should be solved without violence. Unfortunately, those types of solutions take a lot more time and effort, so I totally understand the desire for a quick fix, esp. for those who are suffering.
So while I think Brian Thompson deserves sympathy, I think Luigi Mangione and everyone who supports him does as well. Likewise for both Trump and those who support the attempted assassinations.
significant ideological disagreement with my own beliefs
I don't see what this has to do with watching a film with him in it. I have deep ideological disagreements with a lot of popular content, but I watch it because that content has nothing to do with that ideological disagreement.
I'll certainly boycott things that are directly related to my ideological disagreements (e.g. I avoid Nestle products due to their unethical sales tactics in Africa), but I'm not going to boycott something just because someone I disagree with is involved (e.g. I'm happy to use Brave despite completely disagreeing w/ Brendan Eich about same-sex marriage).
Watching a Jack Black film doesn't impact Jack Black's public views about Trump's assassination attempts, the two are completely unrelated. Refusing to go to a Tenacious D concert if Jack Black decides to go on tour w/o Kyle Gass could have an impact though.
If violence against a corporate ghoul, fascist, or nazi saves more lives in the long run, it is 100% justified
I absolutely disagree. The ends do not justify the means.
If we condone violence as an effective means to achieve political results, we're literally supporting terrorism, because that's what that is:
terrorism - The use of violence or the threat of violence, especially against civilians, in the pursuit of political goals.