Should be the people who decide. If I decide I fancy this as an option I'd want to vote for it or not as the case may be. If you were pro assisted dying why would you want to risk your MP voting the other way?
You could say that about every single decision an MP makes on behalf of their constituents. Democracy at work, innit. There a one or two folk that don't like Starmer or Boris or Rishi. Letting 'the people' decide is great, until you have something like Brexit.