Ursula wasn't really that evil, though. I haven't seen the entire movie, but I did see a paraphrased stage performance. In the play, and in the actual Poor Unfortunate Souls song, she gives Ariel a clear explanation of what she is being offered.
No she doesn't. She doesn't clearly explain that she has full power to interfere with ariels mission nor does she explain explicitly what will happen to Ariel should she fail. Obviously Ariel was a little naive to begin with but Ariel had sinister intentions.
Also, i think the last "Ariel" was supposed to be Ursula.