So him dying at some point shouldn't really have any effect. You can't breath a sigh of relief that he's dead, because he still might turn up and stop you. Or save you. You might turn out to be the first person he ever met, thousands of years after he "died".
He's had a millennia to go anywhere, anywhen, which weaves him so intrinsically into the fabric of space time that even his actual death won't remove him from the universe or stop him from turning up on your doorstep.
Sure, but having a lot of time to do things isn't the same as having an infinite amount of time to do things and, more importantly, knowing what to do and when
. He's not omniscient & omnipresent.
Kill him (permanently) before he has a chance to foil your plans and assuming he doesn't have the opportunity to tell a younger version of himself (or anyone else) how to stop you, you win.
I mean, a younger version might still stop your plans further down the line, but that would be unrelated/coincidental/bad luck rather than due to the supposed pointlessness of killing him. It's still worth trying to kill him when you encounter him, because you have at least a chance he doesn't
eventually turn up in your way in a younger guise.