I don't think this was really all that damaging to Namor. If it were a different character, like say Peter Parker I might feel differently. But Namor is the sort of character who I think would murder billions of people to save 2 entire universes (minus the billions he killed but who would have died anyways).
A Peter Parker won't accept murdering billions of people... but not everyone is Peter Parker. I think there are SOME people who will look at the cold hard number of lives saved versus lives lost, and will act to preserve the greater number of lives regardless of the cost. There are some who will act that way simply because it makes the most sense, regardless of the personal cost.
Strange murdering the Great Society is a bigger issue. He's no Namor, but he's also no Peter Parker. He's somewhere in the middle so it's a bit more damanging to him.