Of course it is hard to prove that someone is doing just that (online, that is, especially); toying with others (for the sake of toying).
And arguing doesn’t, in itself, have to be bad, or done for ‘toying sake’.
After all, one is arguing with a someone if arguments are used doing so, I think.
As for people who do indeed toy with others (knowingly get the other’s person’s panties in a twist): typical bully behavior, I would think.
Maybe they come from an environment where this behavior was a standard.
Maybe they haven’t learned to treat their neighbors as their selfs (selves?)
Or maybe, in some cases, they know exactly how normal discourse should go, but they get a kick out of it.
I must say I recognize the urge sometimes, especially with someone I don’t agree with, or someone I dislike, but I refrain from it (as much and as good as possible).