First time I ran Taral V when my Shadow tank dinged 50, I had a nightmare. I didn't know the skips and people raced ahead. I didn't know you could skip the final boss because I'd never been there before and despite having told the group, they let me charge in and aggro the boss while they all went up the ramp to skip him and let me die. The group was really rude to me too, calling me stupid. Sorry but if someone tells you it's their first time doing something and they die because you don't tell them something they need to know, it's you that's stupid. It knocked my confidence a lot.
Sorry that happened to you. Bad behavior to fresh 50s and new players has become so common in LFG groups, it is astonishing. Especially in Lost Island, but that is another topic.
For me, I never mind fighting the bosses or explaining the fights to new players, but I DO want them to say they are running it for gear or for the first time. And if they are running it for the first time, I expect them to never run in front of whoever is explaining the fights (usually me) or leap into battle without checking chat for a "wait".
The only bosses I will generally refuse to fight are the bonus ones.