I was under the impression that Elan was the most "dangerous" Forsaken, and the most trusted, because he did not really do it for profit or gain.

He think he had concluded that as time pass, the number of times the Dark One try to break free reach infinity and thus the odds that one of the times the Dragon (or whoever is fighting) will fail also reach infinity. It was basicly just a matter of time until the Dark One won.

Why prolong the fighting, the suffering and the agony?

Checking this against the WoT-wiki I find this almost word for word: wot.wikia.com/wiki/Ishamael

"He was unique among the Forsaken as being the only member of the group who did not join for the promise of power or immortality (at least not per se), but because his logic concluded that the Dark One must inevitably win against the Dragon, and therefore the only path was to join him and rule for a time."

@Gwenifer

It have been considered that his "madness" and talking to LTT is just him developing a split personality and he create LTT in order to cope with his sudden extra memories of past lives. All the weaves and memories that Rand would never know, but LTT would know, is just the memories Rand apparently already had since very early on (as mentioned earlier in these comments).

Basicly, he already had all this information, he just did not accept it and fought it.

Apart from that, there is nothing that could not be considered "normal" insanity, considering the pressure the man is under.

Just look at a sulky kid and Rand's actions are not that far off sometimes. Other times he is just acting the way he think it is ment for him to be. It is like looking at someone steeling him/her self after a dear relative have passed away, and you know that they are hurting so badly inside, but whatever you say just cant get through to them because they are so sure that this is the way it must be.

Thus, it is quite possible to say that The Eye inoculated him from the very start and that the only madness he was subjected to was what he channeled before The Eye.

However, he do mention the oily and sickly taint on Saidin even after The Eye, so clearly he sense it, but is he affected by it the way the others are?

Since he cleansed Saidin before Veins of Gold, we will never really know how him having that protection would affect his perception of The Taint.