RE: Given that we have no free will noone should be held morally culpable for anything.

No.  Our desires and proclivities are determined by genetic and environmental factors over which we had no control.  For example, factors include upbringing, social conditioning, the micro structure of our brain and its capacity for empathy.  I’m arguing that a serial killer was born with a serial killer’s brain, just as a lion is born with a lion’s brain.  Both are geared to commit immoral acts.  Just as it is not the fault of the lion that it goes on to commit infanticide, it is not the fault of a clinical psychopath if they go on to commit murder.  Would you blame someone with a stutter for not being able to speak fluently? No, it is a problem in the brain. So how can you blame a mass shooter if that is what the environmental and genetic factors that makes up their life led them to do?  

Take any action.  Eg going to the beach or going for a forest walk.  The decision is not yours to make.  You will make a choice, but the choice is determined by your preferences which are genetic (perhaps you overheat easily so you prefer the water) and/or environmental (say you heard on the news that there is a new walking trail opening up).  You did not choose your propensity to overheat.  It was by complete chance that you overheard about the new walking trail.   

You can go even deeper down this rabbit hole.  I did not choose to be born human, so how can I be asked to take ownership of all of my human idiosyncrasies and behavior?

Lets go deeper.  ‘I’, ‘self’ – to what extent can ‘I’ dictate ‘self’.  I suggest to no extent.  I am a product of nature.  Like a stone, like a gust of wind.  I am capable of choice, but my choice was predetermined by my form and the context that I find myself in – neither of which I control. 

I eagerly await your reply as it is an issue that I struggle with often.