If someone says that Boris's inaction is responsible for thousands of deaths. Blame Jeremy Corbyn.

Well, that's a slight improvement on your previous statement:
The "fact" that the current PM's inaction has caused thousands of deaths does matter.

The current PM has 'caused' no deaths. Whether earlier action would have prevented some of the loss of life is debatable, but the cause of the deaths is the virus.
Inflammatory comments help no one.