A traitor was a double-edged sword to use; you will never know when a disloyal dog might turn his back on you and wag its tail again at their former master.
Although the former master was unlikely to take the disloyal dog again, with enough chips on the table, there was hope of preserving his life in the end.
That was... if he survived. He knew far too much, and the leader would be uneasy about letting him run around. A dead Duke was more beneficial for him, and it wasn't that Spade could see any benefit from saving Duke Merphen either.
Most of the things the Duke knew, Spade was already aware of. And those things he didn't, he could just find them himself.
And now, rather than killing the Duke themselves, what was better than him being killed by the master he served after betraying the empire?