What to do when you hear many bad stories about your supervisor on the first day of your PhD? Today was the first day of my PhD. It's a very prestigious institution and I was extremely excited to start. However, it seems there is a general feeling of discontentment with people in the group and I was told some stories that, quite frankly, shocked me to the point where I was legitimately considering packing up then and there.

I was told that the supervisor of this group has extremely suspicious working practises, blocks papers from students to give the credit to 'preferred' students, has insanely unrealistic expectations (I was told that some students were expected to work through the night regularly), blocking people from graduating to get more cheap labor out of them, and many more things. The term "psychopath" was thrown about a lot.

I have recently completed my master's in a group where there was an extremely poor PI with an incredibly abusive and parasitic attitude towards the group. I don't really wish to go into it because I want to maintain anonymity, but suffice to say that I do not want the same experience in my PhD.

As you may realise, I'm extremely concerned that I've made a truly terrible decision and I would like some advice going forward. I realise that this is only my first day, and I am not going to make any rash decisions without more context and experience, but what should I be looking out for? How do I protect myself from abusive behaviour? And, most importantly, how do I decide when enough is enough and I should cut and run? 

I would really like to get the PhD as I'm extremely passionate about the subject, so in order keep a science career possible I think if the option to quit is on the table, it would have to be in the first year. I don't however think that a career in science is worth the exchange of the toll on my mental health that an abusive boss will cause.
Based on my experience because I had the same situation before and regret not doing that right decision since earlier time. I do recommend you first to stay for couple of months and try to figure out how this PI behaves towards you, in the same time try to look for another good lab. I am still burning out as since month ago this what happened to me, you can see my post here: Forced to Quit PhD although the hard and proved work

So, I dont want you to be in the same situation, I really appreciate that you are passionate about the subject, however, if you have a poor supervisor S(he) is going to ruin every thing.

Another advice and really important which I regret not doing it is asking the current students and alumni, don't be lured by citation and their prestige and trust your gut.

I hope you can take the right decision and for sure I don't recommend you to leave, however stay, work hard as much as you can and if you have noticed red flags from s(he), you can switch from the lab.