Colon keeps being careless.
I'm surprised he did not attempt to take down the stepbrother but instead chose to run away.
I can't understand why the other parasite was so confident though. Let's assume it had succeeded in inserting itself into Stella, what he'd find there is Colon who has already lodged itself throughout her body. Spreading the tentacles, and getting acquainted with the host body, is something that takes time. I would assume that what he tried to do was like charging at a trench and somehow believing that he could dislodge the enemy soldier there. I would expect Colon to be able to draw more nutrients from his host and to completely isolate the other parasite.
Then there's the fact that the dog died to the tentacle trap. I would expect that, when it's not weakened by not having a host, the other parasite's tentacles can be just as tough as Colon's. So if he has spread his tentacles inside the dog, he could have protected it from getting sliced up, but it hadn't done that.
It's still better than being without a host. It can run away and find another host now. Even if he can't move too quickly, Stella doesn't want to kill family.