I think it depends a lot on the execution, some in particular i like are:
1. They're suicidal about it and you can see how much of a destructive mindset it is
2. Their "altruism" coming from not fully selfless reasons (boosting low self esteem, mending a past mistake, etc)
Having your main character fatal flaw be "self-sacrificial" is kind of like when during a job interview they ask you for your biggest weakness and you say that you work too hard