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 unpredictable. 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 and the characters are simply overly fragile, 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 a character being killed by firearms would be unimaginative, and therefore will not be done).

Killing
Obviously, in Happy Tree Friends, death can happen in multiple ways, it can occur through a series of accidents, or on purpose, it can be caused by other characters, the environment, or the dead character themself. When a suicide occurs, such as Petunia in Wishy Washy, these are not kills, and are, therefore, not counted in the characters' kill count. The reason behind this is because kill and death counts are meant to be comparable, and suicide, by a different logic, would count as a kill and a death, which doesn't allow a fair point of view on a few characters. Indirect kills also do not count. Essentially, a kill on the other characters is a kill, a kill on one-self is a death, as a kill here is defined by killing another character. Handy, Sniffles, Flippy, Splendid, Pop, Shifty, and Lumpy are the few characters who killed another on purpose.

Plot Continuity
Many people ask why the Happy Tree Friends can keep on dying, and, despite wikia being 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 is simple, the first point is so the characters can keep on 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. Another theory is that, as shown in A Change of Heart, I Nub You, and Spare Tire, they have an extreme medical system which could heal extreme injuries that would have been fatal in reality.

Permanent Deaths
That being said, there are a few, permanent deaths, and this usually for the reason of some Happy Tree Friends series and/or characters to allow a loose story, and it also allows for the best comedy situations to exist, such as Pop and Cub (mentioned below). One example is Sensei Orangutan's permanent death in Ka-Pow!'s Buddhist Monkey series so there can be a small background to Buddhist Monkey. Another is Pop's wife's/Cub's mother's death.

Permanent deaths are rare, but a few do exist.
 * 1) Pop's Wife/Cub's Mother - Tombstone seen in Can't Stop Coffin and confirmation given on August 22, 2015.
 * 2) Sneaky- Dies from Mouse Ka-Boom's explosion. Said to return in future episodes, although Ka-Pow! has ended production, so it is unknown whether he will appear in the regular series.
 * 3) Mouse Ka-Boom - Dies from Flippy's machete. Said to return in future episodes, although Ka-Pow! has ended production, so it is unknown whether he will appear in the regular series.
 * 4) Tiger General - Killed by Flippy in Operation: Tiger Bomb. May return if W.A.R. Journal gets a sequel, but currently Ka-Pow! has ended production.
 * 5) Sensei Orangutan - His tombstone is seen at the end of Three Courses of Death as verification.
 * 6) The Rat - Dies in the explosion (debatable, may return if a sequel to Mole in the City is made).
 * 7) Prehistoric Sniffles - Assumed due to the fact that his only appearance is thousands of years before the rest of the series (debatable, as we never see him die and he may have survived by getting frozen in ice, like Cro-Marmot).