The issue here was the wording on his part was poor. If he would have said that maga was trying to prove the shooter was left wing, that would be fine (even if there was nothing to prove, it has already been proven by the FBI).
The implication by the poor wording by Kimmel here is that he used a negative "anything other" and the phrase "one of them" which one easily hears this as him implying the opposite of what true in the situation: that the shooter was Maga.
I get comedians will make dark jokes. I remember some doing jokes around 9/11. The problem with this is he has a national audience and he's making a political statement around the event, not a real joke here.