My answer is messy and incomplete! I very welcome editing!
As soon as an individual is killed, on might as well eat the dead, but the act of killing might not be for the purpose of eating. We might even say that eating a dead might not only be for the purpose of getting energy and matter but only to clean the living place. And killing might appear in many circumstances. The best way to consider such issue is certainly by using the quite fundamental model of kin selection theory. It fits as much to species that gain energy by eating the dead or not. The difference imply just a minor change in the payoff matrix.
The decision of one (the actor) to kill (and eat) another individual (the recipient) depends on:
- the relatedness of the actor to the recipient (note a relatedness is not necessarily symmetric)
- The competition this recipient causes on the actor
- The cost (energy cost, risk of being injured or killed) of killing
Many species (ants, wolves) might kill or injured individuals of the same species, when for example there is competition for reaching a status in the hierarchy.
I think that the definition of k-selected species is not very accurate. First because there are limit cases. But also because r and k can relate to the function that describes the population growth of a species or might describe the amount of energy the parents spend into one offspring which is not necessarily the same thing. If we think of the parental care, we can think of the lions. When a male win the right to access to the females of another male, he will kill the babies that might be potential competitors to his progeny (because they take energy from the females or because females are not fertile before having lost their babies).
Killing its own offsprings is certainly not less common in k-selected species. You might think it is such a big cost to kill one of its own offspring when one has few offspring but what is important is to think in percentage. Killing 50% of the progeny has the same cost of an r-selected species than for a k-selected species.
If one has a probability of 10% to be able to raise its offspring to adult age. But this probability raised to more than 20% if it accepts to kill an offspring to save energy for the next offspring, then it wins by doing so. THen we might ask: "so would one make an offspring if it intends to kill it?"
Well it does not necessarily intend to kill it. It might need to decide late in the season whether or not it has better to kill its own offspring.
Then, maybe offspring might be used as a reserve of energy and matter for its siblings or for its parents.