While playing Halo 3 and on the mission, Floodgate Master Chief finds a UNSC marine huddling in a corner mumbling that they all turned. The UNSC used normal weaponry to take them out but that doesn’t make sense I thought the flood was stronger. Someone please clarify?
The Flood are stronger. That is why in almost every encounter the Flood will kill and infect marines if the Master Chief or the Arbiter doesn’t intervene.
In regards to the marine you mentioned, I like and wholeheartedly agree with the theory that as the dead marines surrounding him bear no sign of having been infected, the sobbing marine most likely snapped under psychological pressure and killed the squad himself.
Some of them probably just saw it as their next mission and carried on doing what they do best. Others were probably terrified by the sight and began to mentally deteriorate, like the subject in your example.
The same way a marine would see a cliff when driving a warthog.