What is the idiom/proverb applicable to this situation?
When a driver forces his car into roundabout without stopping (as the traffic rule would recommend) just because he thinks he has signaled ahead of time.
The idea is one should stop before entering into a roundabout rather than forcing one's car into it just because one comes signalling from far away. So here the driver is wrong in the first place and over the top he then throw tantrums post event.
A resembling idiom in hindi is
Ek toh (First of) chori (caught stealing) upar se (over the top) seena zori (chest-thumping)