It would be nice if the game allowed for some coverage of the possible (and in fact LIKELY) desire on the dwarf-noble PC's part to get sharp-and-pointy revenge on Bhelen. There are decent reasons why you can't simply murder him; it would kinda throw a spanner into your ally-raising. Harrowmont isn't much for backstabbing and probably would refuse to work with an assassin even if that assassin benefited him. But come on, it's perfectly logical that the dwarf noble PC would work for Bhelen just enough to get access to the guy and then attempt to kill him.
If this were a visual novel, it could give you the option and then explain in HeadText (character thoughts) why that's really a bad idea. Of course, if this were a visual novel, it could just let you DO it and then end the game because you fucked up and got executed... a VN doesn't have the game design principles that insist on pushing the player along paths of the same length towards the same climax. If Bioware weren't so dedicated to same-length-paths, they could just let you assassinate Bhelen, lose the dwarves as allies for the finale, and continue with the game anyway. But that would cut off hours of gameplay from that path because of the Deep Roads! So they won't do it.
But it feels lacking, IMO, to have a good reason to want someone dead, to have guards blocking you from meeting him at first because they're rightfully afraid you'll kill him, and then to not even have the option to express that you WANT to kill him.
It could still be done, even within their constraints. There could be a dialog option upon first getting through to meet him, to choose 'kill him' and then have Bhelen himself catch your hand, point out why that would be a bad idea, but be amused that you'd tried - and plot carries on. Also, there could be slightly different options when a king is finally chosen, to make it more clear that you're backstabbing him at the last minute. Don't we betrayed nobles deserve some personal satisfaction here?