Many suspect Putin is simply angry because his own government's cyber-security was just put to the test and fell a little bit short. Russia has a reputation as a cyber-savvy nation of "computer experts," and they lost a bit of that street cred when WannaCry ripped through their defenses like paper.
Russia's been scrutinized thouroughly as of late: it's widely believed the country interfered not only with recent U.S. elections, but also with the French presidential elections which have just passed. In one case, they got what they wanted. We'll let you sort out which one that was.
In the meantime, however, Russia was among the nations most affected by the malware attack. One Russian official commented on the attacker's motives:
I cannot exclude that the main task consists now of frightening the whole world. The attacks hit hospitals, railroad transport and police. Over these days, the world got a serious warning.