Yes they are doing a great thing but yes they have blood on their hands.
Are they doing a great thing?
While I don't disagree with their cause, I'm not convinced that their actions help rather than hinder its realisation. I suspect public opinion is largely in favour of decarbonisation to help climate change, so in that context I'm not convinced you need to take drastic action to get attention - the attention is already there. The risk is that you push climate change back from being a mainstream concern with the public, towards one that becomes associated with a rather extreme group of actors.
I get that there is a time in a campaign when you need to bring things to the attention of the public and sometimes need to use quite extreme methods to do so - but on climate change that time was perhaps two decades ago, not now.