Met Police arrested the suspect on suspicion of murder. After investigation, he was charged with manslaughter.
Now, go and look up the differences between murder and manslaughter (note: they're fundamentally the same between the UK and the US), then apply what you've learned to the details of this case you have privy to that no one outside of the police and the CPS do at this stage. I've checked five different news outlets on this offence, including the Met Police's own public link to the case above, and as this case has yet to be tried there are of course no details given.
So, what do you know that no one else outside of the Police and the CPS do that leads you to conclude this was murder and not manslaughter?