IIRC the devnotes for when he says that seem to imply he's being sincere when he says that, it said something like "a little bitter, but more a warning than a threat". But whether he truly means it or not is the least of my concerns tbh.

I feel like I'm being too negative so on a positive note I do like that Spawn Astarion's epilogue (if Cazador's dead, that is) seems quite genuinely happy. Props to Neil's acting.