Learn English – “I experienced a feeling I have never had before” – is sequence of tenses correct

1. We use the present perfect to talk about an experience at any time in the past. The exact time of the experience isn't important.

   I've seen the latest Megan Fox Film. Have you ever been to Spain?

2. We use the past simple to talk about a specific occassion in the past.

   I saw the latest Jim Carrey film last night. Did you go to Spain last summer?

So, in your example, the tense is,

I have never felt appreciated in my whole life.

He had seen them hurl themselves at their enemies until the ground was piled high with their corpses … and their opponents had depleted their ammunition.

One way to look at it is this:

The switch to the past simple here (until the ground was piled high with their corpses) hinges on the word "until," which is a ... hmm ... a time indicator. Time establisher. Whatever. It's in the same category as "when," "before," "after," etc. You're saying it should have stayed in the past simple ("until [...] their opponents depleted their ammunition"), but it would be wrong. The past perfect here is used to show that, in fact, the opponents' ammunition was, in fact, depleted. "... until their opponents' ammunition was depleted."

Another way (a more complex one in this case, so don't pay it much heed) ... another way of looking at it is as follows:

Once we've established that the action takes place in the past perfect ("He had seen"), the rest can continue in the simple past. (The explanation about using the active voice rather than the passive ("... they had depleted ... " instead of " ... was depleted ...") is the same as above).