"So if someone asks me to punch them, I oblige, they die, then I’m not responsible"
Punching, quite possibly so. BecUse it all comes down to intent, and thereis a level of (what could otherwise be assault) one is 'allowed' to consent to. But if I all ended up in court, then it would be a a casei of whether the person had a reasonable belief that the action was wanted. So for example, a couple who had met on a BDSM site should be able to have the sex of their choice without fear of prosecution.
There is however a level of harm to which no one is permitted to consent. The infamous case of this was in Germany (Armin Meiwes, the Rotenberg cannibal) - where consent was known to be given, because of the actions both and videos of the events.
Strangling deaths do occur from time to time.
Sex with props is terribly common. But with a knife?
Has anyone read the full sentencing embarks from he judge? They ought to show quite clearly what the court heard.
What does it mean he did not intend that to happen. What did he think would happen"
That would be found in sentencing remarks. The press report is that there was no suggestion that the violent sex as not consensual. That's quite a strong statement, and presumably evidenced (eg witnesses, calls/social media use at germane times, conduct around the time of the incident, and in this case I guess evidence of the nature of the relationship - including anything about the sexual aspect that either of them had discussed with third parties)
The role of intent is important in murder, and part of he definition in English law. There has been huge consideration of how far one is allowed not to intend the obvious outcomes of actions (if you want a good but dated literary version, try 'Thrones, Dominations' by Dorothy L Sayers, where it's an element in the plot)
From that press report, I cannot see why he was not tried for murder. Nor why the sentence was so low (manslaughter sentences can be considerably higher). A full version of the remarks might be illuminating.