By whether they're making verifiable and objective claims? They aren't, really, and they certainly didn't source theirs, but it's a lot more convincing to say that instead of making your own unsourced accusations.

I think the issue is that you've not provided evidence that the other user supports Israel's genocide in Gaza.

At the end of the day it's just an internet argument and not worth it, but if you want to make your point, I'd start by getting that evidence.

I do agree in principle that I would think twice before agreeing with any position a genocide supporter takes, though that doesn't necessarily mean I will disagree on any particular point after giving it due thought. If they said throwing puppies from a roof was bad, I'd agree, for example.