Aggressive and Thief aren't the same thing.
The Thief flag means you stole from someone, and it forces you to be Evil or Neutral. Nothing more.
Aggressive means you attacked someone, and it's similar to being Evil.
If you steal from someone without attacking them, you're flagged as Thief, not Aggressive. On the other hand, if you attack a Neutral player, you will be flagged as Aggressive.
From what we know, the only way to steal from someone is by attacking them until they are knocked out. The act of attacking a good or neutral player will flag you as aggressive, and stealing or murdering them afterwards will extend that flag. I don't know what would happen if you stole from someone who's already knocked out, but it would only make sense to flag that person aggressive as well to prevent players from cheesing the mechanic.