I think that any group of people has a right to remove themselves from any other group of people. The question is really what territory they take with them, and if it's just the territory where they live. It seems (and it's definitely far from clear) like the Russians recognized Donbass to include substantial territory that had not seceded from Ukraine.
I feel like the Ukrainians could have avoided a lot of this mess if they had just said 'yep, we recognize you guys as independent, and the border is the current "line of contact".' That also would have allowed them to join NATO (since having a current border dispute precludes entry).
There are a lot of lessons here for Moldova, if anyone there is watching.
The Russians have done a great job of using NATO's own rules against them. NATO won't admit any new members with border disputes, so Russia foments a rebellion and has the border region secede. The country (Ukraine or Moldova) refuses to recognize the secession, and is hence barred from joining NATO.
The real solution would be to just recognize the secession, let that region go it's own way, and move forward with the people who actually want to be part of your country.