Kill a 2nd alien?

Its 2027, I'm pretty new to the game, its on easy mode, and I have completed the "retire an alien counselor" mission. I've played conservatively so far and, besides bumping off a few vulnerable command nodes, its the first time I've harmed an alien asset (human collaborators included.) So the threat-o-meter stayed at 2 and I'm wondering if I should go ahead and retire one of his friends too. I have 12 mission control nodes and 4 habs, with plans to build a 5th.

It seems like killing another alien would slow down their mind-slaving operation, which is most directly threatening humanity, without provoking an alien counter-attack, so that's a good thing. On the other hand, I know I'm going to have to shoot down the alien surveillance ship and I don't know what else before I can seriously contest control of the Earth-Luna system with the invaders, so maybe there's no margin for the additional provocation? Please advise me!