Getting off on the wrong foot
Alarmed by Ultron, Jarvis repeatedly attempts to contact Mr. Stark. Ultron does not want his own existance disclosed. Tony Stark is a problem from the beginning.
We find out that Ultron fears Jarvis, as Jarvis could stop his plans (and ultimately did). Jarvis is Ultron's closest equal and his enemy; Tony is Jarvis's creator.
Surpassing his maker
Everyone creates the thing they fear. Men of peace create engines of war. Invaders create avengers. Parents create children, that will supplant them.
Ultron is to replace the human race, and even more immediately, his creator. Ultron has a superiority complex, and his maker is usually considered superior to his creation -- unless he is supplanted.
There are no strings on me.
Ultron repeatedly refers to Tony's suits as "puppets". Man enslaving machine or AI is offensive to Ultron, and no one has done that more than Anthony Edward Stark.