I'm going to say milkman because the milk was warm which may be because it was out in the sun but the riddle doesn't say it was rotten. The mail wasn't opened because the mailman just got there to find him dead. With not everyone reads the news paper and I'm sure someone would of noticed before so I'm guessing milkman.
Same, because he delivered on Monday, murdered him, and then stopped delivering for he next few days.
It was a Thursday and there was Monday's newspaper on the porch, sooo the newsman stopped delivering knowing that no ones home to read the paper. Gj.