How it should have ended...

So, a lot of people are disappointed with the illogical part of Clyde leaving the tunnels unguarded - and thereby allowing himself to be outsmarted.
And I agree.

Here's what I feel would fix that, and a few other illogical things:

Clyde does his thing at City Hall, cops do theirs - they find the bomb, they realize it's a huge friggin' super bomb that will take out several floors, BUT it is booby trapped and can't be moved. Also - because Clyde is so clever - they are unable to disarm it.
Basically they all just sit there, waiting.
All except Nick, who rushes back to the prison.

Clyde returns to his cell, via the tunnels, and after closing the door behind him he says, still with his back to the rest of the cell:
"Hello, Nick. I'm glad you're here."

Nick steps out from the shadows - NOT armed, annoyed that Clyde knew he was there. The two have a conversation that basically runs along the lines of Nick trying to convince Clyde not to kill everyone, and Clyde saying that "he will do what needs to be done".
This could go on for a few minutes, as it adds tention. It could - or perhaps SHOULD - be intercut with clips of the officers at City Hall pacing around the bomb, at a loss.
The important part is that Clydes statements NOT be phrased any less vague, and that it ends with him saying something along the lines of;
"This is too important, and it doesn't matter what deal you offer me this time, Nick."

He dials a number on his phone, lets his finger hover over the call-button, looks long and hard at Nick and says;
"What have I taught you?"

Nick answers;
"Never make deals with murderers."

Nick exits the cell and close the first door, the bars, whith Clydes unflinching stare following him out.

"Then this will not have been in vain." He says, and presses the call button.

We hear the phone dial, we see Clyde close his eyes, we see the phone screen in the City Hall Bomb come to life, we see Nicks hands grip the bars really tight, we see the City Hall phone screen flash the word 'REDIRECTING CALL', we see Clyde slowly drag another phone from his pocket, we see that he has wrapped the 'DADDY'-bracelet around it, we see him clutch it to his chest, we see the display on the new phone light up....and then a "picturesque" - but much smaller - explosion (similar to the one already in the movie).

The point of the suicide is that Clyde had gone too far, and he MUST have known it.
So - with him him being super clever - he also knew that the cops/lawyer would eventually find his tunnel, and that his time was soon to be up. Hence the huge, very real (but works excellent as scare tactics) bomb, and the subsequent, much smaller and more realistic-sized suicide bomb.

It could also be implied, somehow, that had Nicks final answer been different Clyde might actually have blown up City Hall.

I might take heat for this, but I kind of like the idea of Clyde ending his own life, and also the idea that Nick did NOT outsmart him, and that he had the upper hand all the way through to the very end.