I don't know that I'd argue the rushed bit but here is how I saw it not being contrived.
The mask represented a type of slavery, it was an instrument of submission and to be seen as an example. We saw this at the start of the movie. So at the end when CC turns McCulen's face into that mask its a way of enslaving him. Making him an example to the rest of what will become Cobra that he,CC, is not to be taken lightly.
Was that moment rushed, perhaps but I say not contrived. It served its on purpose within the internal logic of the film. Now had CC strapped him down and given some windy speech about loyalty yadda, yadda and then injected him we'd be talking about what a cliche mustache twirling moment it was. It had to be done one way or the other.