Quote
It is not Seths sword, you never even see him die, how can you conclude that it is his at all???

It is as Barta says, plus you can see Seth, go to the sword and then go back to the alive and well Seth.
So no, it is not Seth's sword.

I am in blood
Stepp'd in so far, that, should I wade no more,
Returning were as tedious as go o'er.