There's a quote running around the internet that basically states that the road to hell is paved with good intentions.
Is this really true in real life, IE does a large share of negative impact come from good intentions?
And if this is correct, why does this happen?
Sometimes it is, but I think more often the road to hell is paved with bad and/or selfish intentions. For instance, the Nazis weren't sober-minded radical altruists who begrudgingly came to the conclusion that they had to do what they did for the greater good – they were instead following an ideology which was racist and evil, through and through. I'm also pretty sure that most people who participated in the slave trade weren't doing so with "good intentions", but instead simply to make a profit.
OTOH, I think the claim does apply, more-or-less, specifically to socialist movements, and I think it's important that other consequentialist ideologies maintain significantly better epistemic hygiene and course-correction mechanisms than socialist movements typically have. But I'd also point out that global movements for democracy, slavery abolition, and anti-colonialism have also been done with good intentions, so "doing things with good intentions" has a track record of doing some really good things as well, and I think on net is a lot better than alternatives.