Latest update is that 6 foreigners are killed (including one Indian national) and that the front right tire burst, causing the driver to lose control and the bus hit the electric pole at full speed. In contrast to the first reports by the police, where they believed that the driver fell asleep and hit an electricity pole and rolled over.
Thai roads are dangerous, maybe more during the night than during the daytime, if possible, try to use trains if you can't afford flying since especially the night buses are dangerous. Unfortunately these kind of crashes with big buses but also with vans occur on a regular base around Thailand, be (fore) warned.
The company that operated this bus is Transport Co., Ltd, apparently a reputable company. It also seems that the bus was actually the bus heading for Koh Samui and not for Koh Phangang. It's possible that the buses in Bangkok were switched and although the side of the bus reads Bangkok - Koh Phangang, it was actually heading for Samui.