There were a few hundred kohanim gedolim in the second bais hamikdosh many of whom died on 'the job' on yom kippur in the holy of holies. Knowing this, what induced them to become a kohen godol which seemed to be for some (the tsdokim) a certain death?
The Ben Ish Chai (I'm not sure where - I was told this by one of my Rabbis) says that in the context of the dispute between the Tsadokim and the Perushim, people would pay to be Kohen Gadol for Yom Kippur, knowing that they were unworthy and likely to die - but they did it for their cause. Both Tsadokim and the Perushim would pay for the office, because they felt that if they didn't sacrifice themselves to do the Yom Kippur service, someone from the other sect would do it, and would do it their way.
This is quite a complicated answer:
One could say they already knew they were going to die. They were perhaps ill or knew through the stars like the 'ish itti' who also knew he was going to die when taking the goat. But then it wouldn't prove like the gemoro wants to that they were 'bad' men.
So my novel answer. Perhaps they had hotzaas zera l'vatala on the previous yom kippur. The gemoro says unless one is a tsaddik one wont live through the year. They knew they were not tsaddikim (but tsdokim!) so they signed up to be kohen godol to be able to die in the holy of holies. It says the angels also couldn't go in there on yom kippur. So he did not have to die with sword of the angel of death and since he was going to die anyway its better to die with the hand of hashem alone.
Now if they were tsaddikim they would have lasted out the year and longer as the gemoro says. Since they didnt that proves they weren't.
to FRED. The gemoro in yuma 19b says a malach did enter. The artscroll answers in the name of the yerushalmi it was only if the avoida was being done properly. So that goes against what I have written so far. The artscroll also answers in the name of the maharsha that it was only whilst he was in the holy of holies not when he already had left.