Just because he knew what was in there didn't make it easier to go in the building. Maybe it was worse just because he knew what it'd do.
Just because he knew how to get rid of it didn't mean this job was easy. The thing was evil, it wasn't going to help him find, salt and burn its bones. It would fight him just because it could.
Just because he saw his brother fall, he was going to finish the job. His brother's shouting 'I'm okay' meant nothing just because his brother was good at pretending everything was fine.