In the past when going through the part where Toguro powers up to 100% over 100%, I always assumed that he achieved this power up simply through some sheer will force or something of that variety. But this time around I suddenly got a new idea as to how he made this power leap and I just wanted to bounce this idea off some of you and see what your guys' take may be on it. The idea goes like this:

1. To achieve this power up Toguro uses his life energy, much like how Yusuke and Kurama have done so in the past.

2. The reason Toguro would do this is because he knew he was beaten, and he wanted to go out in one last blaze of glory. This notion would actually keep in character with him as well because it's assumed in the end of the arc that he wanted to lose to Yusuke anyway and also since his whole life was about gaining power, he wanted to use this chance to push himself to the absolute limit and as a bonus he is able to take Yusuke's full powered Spirit Gun, as a last testament to the power he gave so much up for.

Edit: Plus if you take all this into consideration, you can only come to the conclusion that Toguro died because he used up his life energy and not because of being spent from Yusuke's Spirit Gun