Pathfinder Antipaladin DMPC

[Stormageddon]

I know that it is not a good idea to play a Paladin DMPC, but what about an antipaladin DMPC? I DM for a small 2 person group, and have been using a Druid DMPC in a party of a Witch and a Dread Necromancer. So far the damage output has been unimpressive. The Players have been debuffing enemies into the ground but not able to finish off enemies quickly. Combat has been taking forever. The players expressed to me that they want to join team evil. So I was thinking that I could give them a Dhampir Antipaladin bodyguard who wants to become a full vampire (the current head of team evil is a vampire, so it works story wise.) Is this at all a good idea? What are some pitfalls I should avoid in doing this?

[Thump]

First things first, you'll want to make sure he doesn't end fights in one or two rounds. Once he starts doing that, your players will feel useless.

Try to keep his CHA and STR as high as possible, without making him one-shot everything. Don't give him a crit fishing weapon, but something like a greataxe or a lucerne hammer. Greatswords work too, but for the love of god, don't use a falchion. I use one on MY antipaladin and he seems to be ending encounters on his second turn. (He's only 5th level, and putting out 2d6+16 per round, he uses a large keen falchion, with a 15-20 crit range.) However, do give him a couple of neat little tricks without overpowering him. As he levels up, you can sort of develop the vampire template, like a Savage Progressions sort of thing (I don't remember the name)

Hope this helps some.