Yep, I've killed him but why can't I trap his soul! I want his soul as one of the leader who's stood up for his people.
Now I just need to put his light out to end the bloodshed of Skyrim.(I regret killing him but he rebelled at the wrong time)
The general Tullius get the honour to the killing blow.

I think to soul trap, you need to be the one performing the killing blow.

He's gonna be pissed.

I'm pretty sure that once the trap has been set the killing blow can come from anywhere as long as it happens while the trap is active. The only other requirement would be an empty gem of the appropriate size, which must be in the inventory of the person who set the trap.

This.

My character uses a bow with a soultrap - often she hits them with an arrow, then her follower tank in and kill them, and the soul is trapped.

Bingo. This is why I always use the longest duration possible for the trap when enchanting a bow with Soul Trap. It allows more time for a follower (if I have one) to finish them off without the trap wearing off first and without me having to put another arrow into them every 1-2 seconds to ensure that it stays active.

For melee weapons I always go with the 1 second trap since I want it to reset every time I strike them, but with bows it's just the opposite.

I have a black soul gem but it didn't work!