Bucky was brainwashed by Hydra! Why the hell is that so hard to understand?! Sure he had the right to be angry for not being told before but he had no right to try to kill Bucky. STOP JUSTIFYING TONY!
The trouble is that if you are going to justify it, you also have to say that Zemo trying to kill Tony to avenge his family was right as well. It can't just be OK for *one* person and not for the other.