Interesting question, I don't have a definite answer, but it does seem to be a flaw. Most parasites I know of tend to keep their host alive rather than kill them off, like the parasite that replaces the tongue of a particular fish or different kinds of worms, including tape worm. At least I've not heard of these parasites killing their host.
I know there is a parasitic fungus that grows on insects and takes control over its nervous system to move it to a spot where a predator like a bird can eat it. This does seem pretty stupid, but the advantage for the fungus is that if a bird eats it, its able to spread itself once it becomes bird poop. I think there's also one that takes them to higher ground to spore. I am zombified and my brain isn't functioning, so I can't think of other examples I've seen in documentaries of where a parasite will kill its host.
But it may also be that the parasites needs more nutrients than its host can provide in order to stay alive. It's a guess, but flaws can happen because they've not fully adapted to the environment but have adapted enough to reproduce. You might view it as a transition period in their evolution or in their adaptation to their environment or like in the case of the above example, they really only need to reproduce - it fairly common in organisms that once they've reproduced or lost the ability to reproduce then they die, so it might not matter that they kill their host. Heck, the fact female humans live for many years past menopause is an anomaly, from what I understand it's something that doesn't happen very often in nature and it's a trait we share with whales too...assuming I've got my facts right there. That would be my educated guess on this one.