Problem fixed, thanks to ArmyUnit's assistance ingame.
The issue was that the bug team had no players on it so when someone died or left on the marine team, slayer ran a check for how many people each team had, and since there were no players on the bug team, it assumed that all the "players" on the bug team had died, and so it awarded victory to the marine team.
The solution was to set the preferred number of players on bug team to 1 and have it be a fake bug bot.
Locking