The meme uses the "man checking oven" template to mark Trump supporters as stereotypically "gay" and contrasts this with a picture of the assassin in which there are no stereotypical signs of sexuality, which of course implies straightness in our heteronormative society.

Sure it is entirely possible the assassin could be gay IRL by coincidence, but this isn't a helpful for understanding or interpreting the meme, since contrast between gay and straight is clearly created and this contrast is used to make a normative claim, i.e. Trump supporters are gay (i.e. bad) for being hypocritical, while the CEO killer is based by living up to the Punisher anti-hero vigilante ethos and thus not gay (i.e. straight, normal, good).

Maybe the meme uses homophobia because it will upset homophobic Trump supporters more, since they don't want to be associated with being "gay". Still, it appeals to and uses homophobic logic by associating the marked-as-gay traits with something villainous and the "unmarked" (which is the heteronormative default of "normal" straightness) with a hero.