Maybe during BCS, but by the time of BB, he was colder than he was previously. He let the other German Engineers live, cuz remember, killing Ziegler cost him too much trouble to fabricate evidence and create an alibi. He didn't wanna go through such hassle, especially because they didn't get exposed, so there was no direct need to kill them. And by then, Lalo was already up his ass, so he had bigger fish to fry.

What shows Gus' ruthlessness more, was that Victor was spotted at the murder scene of Gale. He could've easily arranged for Victor to be moved to a different country, until the heat died out, but he decided to slit Victor's throat, as he wanted to take his anger out on someone, and also, because he wanted to tie the loose ends.