Meant to respond to this ages ago, but Voldemort is credited with mastering every form of magic besides Love, and there's some insanely powerful curses.
There's the ones we know he can use, such as the curse that he placed on the Ring, which would have killed even Dumbledore had Dumbledore not been Dumbledore and not gotten Snape to seal the curse into his arm.
Even if Belos some how manages to do the same, the Curse strengthens over time and will kill Belos eventually just as it did Dumbledore.
Just looking to the Unforgiveable curses, which Voldemort is probably the most proficient at in the entire Wizarding World, he can essentially put Belos under his control indefinitely, with even other powerful being unable to resist.
We have only 1 instance of someone resisting Voldemort's imperio at full power (I'll explain in a moment) before and that's Harry, who had specially trained to protect himself against it that very year and was empowered by his Mother's love (which still worked, even after Voldemort got Harry's blood) and a portion of Voldemort's power. The only other instance by a Non-Plot empowered character is Bartie Crouch, who is 1. Noted as one of the most powerful Ministry Wizards of his time but also was only able to resist it during brief moments and 2. from a Voldemort who could barely move around. Belos resisting a full strength Voldemort's imperio is laughable, and as the fake Moody points out, you can have someone kill themselves with Imperio;
Everyone was laughing — everyone except Moody.
“Think it’s funny, do you?” he growled. “You’d like it, would you, if I did it to you?” The laughter died away almost instantly.
“Total control,” said Moody quietly as the spider balled itself up and began to roll over and over.
“I could make it jump out of the window, drown itself, throw itself down one of your throats . . .”
Moving onto more esoteric things Voldemort CAN do, but is never shown to do is the curse placed on the Pendant from Half Blood Prince, which by grazing the cursed object with a tiny hole in a glove you are put into a near-death state so bad that even trained Medical Wizards have difficulties healing you.
Dumbledore themselves flat out say if she held it in their bare hands, it would instantly kill you:
“Still very unwell, although she was relatively lucky. She appears to have brushed the necklace with the smallest possible amount ofskin: There was a tiny hole in her glove. Had she put it on, had she even held it in her ungloved hand, she would have died, perhaps instantly. Luckily Professor Snape was able to do enough to prevent a rapid spread of the curse —”
Likewise, there's also Fyndfyre, cursed fire that's so destructive it breaks things beyond Magical repair, literally regeneration negation. Hermione thought it was so dangerous that she didn't even consider it an option when destroying Horcruxes, and that's saying something considering their options was the Sword of Gryffindor and literally nothing else.
Anyway, all this to say, it's hilariously in Voldemort's favour through sheer quantity of ways of kill Belos
