Kes killed Tuvix, because she missed Neelix.
Up till that point they were deeply in love and there was no signs of cracking.
Gosh, then here's the snuggling and cuddling while they watched 20th century TV together in Futures End, and there was definitely no signs that there was anything wrong with their relationship... The very episode before Warlord.
Although Fair Trade is the very episode before Darkling where Kes seemed free and clear to sow her oats.
Maybe it was the drug dealing?
But then what does Kes know from Drug dealers?
The concept of a criminal like that who kills to maintain the supply of product that slowly kills his customers who are addicted and can't help themselves must be ever so alien?