>>535553336
You're wrong, and retarded.
A truly evil Servant would still fight for you, because if he didn’t, he’d be nothing but a liability. Gudao would terminate the contract and send him back to the Throne of Heroes. To avoid that, the Servant would grudgingly prove his worth in battle while quietly biding his time.
And if he’s powerful enough, that usefulness might persuade Gudako to keep him around despite the risk. He’s essentially a voluntary hostage. Any real betrayal would see him crushed by the 400 other Servants in Chaldea and promptly banished back to the Throne.
Summoning a 100% evil servant could absolutely make sense, and be P R O D U C T I V E to our causes.