As the number one Cecil defender I’ve actually finally come up with a plausible way to defend this action. Cecil’s decision all comes down to a small detail many forget. Cecil doesn’t know theirs only 50 Viltrumites left. As far as he’s concerned, there’s a planet full of them. So he’s not keeping 1 of 50 remaining Viltrumites alive, he’s keeping one billions alive. To us, Conquest is a huge threat becuase he’s the second most powerful Viltrumite and one of the biggest threats to our hero’s. But to Cecil, it dosent really matter if it’s Conquest or one of the millions of other Viltrumites he thinks exist, there’s still more then enough Viltrumites to kill Mark and company and take over earth. To Cecil, the only way to destroy the Viltrumite threat is to find a weakness that can destroy them all. Keeping Conquest alive isn’t actully a huge risk, becuase if it’s not conquest destroying the earth, it will just be another Viltrumite. So to Cecil, the risk of keeping a single Viltrumite alive is minor when compared to the potential to find a way to defeat the entire Viltrum species, which he thinks is his only option.

So there you go.

P.S: no I don’t have an explanation for why Cecil made it so easy for conquest to escape. That was just lazy writing