Somerset pissed at illegal entry

Well yeah, he didn't want his illegal manner of obtaining information exposed. The difference is that it would compromise their investigation as they would need to explain why they would trample all over a citizen's rights. Doe's lawyer would have a field day. Somerset could always claim he got an anonymous tip about Doe.

I was just reading an early draft of the script where Somerset successfully keeps Mills from kicking the door down. They pay a vagrant to swear that they called the police BEFORE hand, so they can get a proper search warrant first. Easy guess they re-wrote that simply because it was a such a slam on the brakes with several extra minutes of police procedural stuff instead of going straight into the apt.