I got nothin'.
If the swing arm is undamaged, it should not be possible for the wheel to go in at an angle... the axle goes through two slots and there's really no chance for it to be out of square.
This view doesn't show the stamped metal spacers that should be there...
... meaning these....
So: If it's impossible for the axle to go in crooked, then there are only two things I can think of that would cause the wheel to be skewed like that.
Either:
1) There's something wrong with the wheel assembly. If all parts are there and the rim hasn't been bent, then the only possibility is the hub... bearings?
2) The whole swing arm is crooked. You said you were "putting the bike back together." Did you take off the swing arm? Could that have gotten put back together wrong?
You can do a quick/dirty test of #2 if you've got a level floor, by measuring the distance from the ground to the bottom of the swing arm on both sides with the bike up on its center stand.
If it were me, I'd take it all apart and make darned sure all the parts are there, check for anything that's not square and put it all back. Not cool to ride with a bike that's deranged.