Really sad. There seems to be a devastating mortality to the Babylon 5 cast. When you look how well the original Star Trek cast have lasted and compare it to B5 it is just heartbreaking. (You can even compare to Deep Space 9) which was on the air at the same time and the disparity between cast deaths it horrendously in B5's 'favour'
Doyle must have been the backbone of the show, he was there from the beginning to the end and developed from the simple chief of security into a happy, wealthy family man, albeit by way of being a pawn of the telepaths and nearly bringing everything down....
He was never going to attain the heights of the hero like Sinclair or Sheridan, neither was he going to be the complex personalities of Londo and G'Kar, in many ways, despite the wealth and power he ended up with, he was pretty much the every man, manipulated by outside forces but still riding things through and coming out the other side.
And the fact it felt like that must have been down to Doyle, giving the show something that every show needs: someone for evey one to empathise with.