A vindicare assassin seems a little... excessive? Like, a space marine scout, a novitiate sister, a bullgryn; Those are all on a pretty level scale, near the bottom of their respective organizations.

Vindicares are sent to execute the Emperor's greatest enemies with a single bullet. When they put together an assassin team to go after Horus, one vindicare assassin was considered sufficient for the task. They failed, sure, but that's still one-sixth of the team sent after a primarch.

I don't mean to harp on about this, but Andreyich was told he couldn't play a greyknight because it was 'too much.' Vindicares are sent to assassinate space marine chapter masters, and usually do it while the target is in full gear.

A ratling seems like the appropriate tear for a sniper in this group. A death cultist would be the right mold for an assassin.
A Vindicare would be like using an earth-shaker round to kill a single chaos cultist. The ammunition costs so much, you could never justify using it for anything of that scale. Literally, anything that would require a vindicare on the team to kill would be so much above the paygrade of the rest of the characters present, it renders having them almost a moot point.

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To be clear, I'm not saying "don't let skwint play a vindicare.' I'm saying, "if he gets to play a vindicare, Andreyich should be able to play his grey knight, and I'll need to come up with something higher up the food chain that an acolyte."