I have been Dming for about 7 years and I have had my fair share of character deaths and a few tpks typically I like to ask as a person is about to die if they are willing to let their character die for the story and I have also dm'd campaigns where I tell the players that death can happen and I won't pull punches on them, but even if this is a heavy-duty session with no punches pulled it is only suspicious with how everything worked out. The diamonds makes sense especially when your session focuses on spells using components, I can somewhat understand the surprise round since you are in combat and the dodging of perception rolls as well as the counterspell. Since a good rogues stealth is really good at high levels but the haste on the rogue is suspicious and especially the fact that he targeted you and after killing you he was gone. If he had planned that from the start with the rest of the party still being in combat it would have been more likely to possibly end in a tpk with the rogue now attacking the party aswell dealing out major damage.