Death and Injury

In reality, death is usually a fairly consistent force, it is predictable. What is different in Happy Tree Friends is it becomes nearly completely unpredicatable. The simple reason for this is, there is no logic to it: sometimes it follows real-world examples, such as Petunia's (possible) death-by-fire in House Warming; sometimes there is simply a lack of logic in the deaths involved, such as Disco Bear's death in Ipso Fatso, where common reactions by the environment are multiplied to create a deadly scenario; and there's the situation where, even by Happy Tree Friend logic (if that's what you can call it), makes absolutely no sense whatsoever, the best example for this is Nutty's death in Swelter Skelter.

Character Death and Injury
The truth with Happy Tree Friends is there is no consistency with death and injury, the Happy tree Friends can be as weak or strong as necessary for the given scenario to play out, such as Splendid, who can survive in space, crash through building, be hit by a car, and get crushed by the moon, apparently dies in a measly classroom explosion. This allows for a huge range of deaths and injuries to keep the show interesting and fresh (and, on the same tone as 'fresh', Kenn Navarro has stated that no character will be killed by firearms, because it would be unimaginative).

Plot Continuity
Many people ask why the Happy Tree Friends can keep on dying, and, despite wikia is supposed to document as much as is possible about everything out there, it's not something that can be covered, because there is next-to-no plot continuity. The reasons behind this are simple, the first point is so the characters can keep dying, and it allows the creators the freedom they need to make this show; also, it means that the characters themselves can become well-explored (and this has lead to fanbases for individual character eg. Flippy). The other reason is, despite this being a wiki, it doesn't need explaining, there have been many fans asking around asking if Splendid and his time-reversing ability is the reason why the Happy Tree Friends keep dying, but he's died himself, and Splendont was trapped in a mirror at the time (most likely), so the simple answer is a lack of plot continuity.