I never advocate attempting in-game solutions to out-of-game problems. And a DM proxy forced on the party is an out-of-game issue with the DM. So talk to the DM about it. I had one group that would do a lot of inter-party (but not inter-player) drama and RP, and the DM said thatr without a DMPC they felt bored and left out of that. And we had to figure out if that was acceptable or not. But generally, I am strongly against DMPCs.
(BTW, it is perfectly possible to have NPCs who may temporarily travel with a party and are not DMPCs.)
If you do accept a DMPC into the group, in terms of killing them - PvP is often discussed in Session 0. If it's okay with the group between PCs it should be just find when that PC happens to be run by the DM. And if it isn't that's an additional red flag that the DM is not only running a DMPC but showing them favoritism.
On the other hand, if PvP is not part of your social contract, don't do it to any PCs regardless of who is playing them. At that point you'd be in the wrong, regardless if you feel others are in the wrong as well.