I'm afraid Tony is in my 'fallen heroes' club, bit like his namesake Fernandes. And you know fallen heroes always receive the bitterest (and probably unjust) criticism from their former followers. I was a big fan as a teenager, based on his book 'Arguments for Socialism' but a few years later when I read of his behaviour as part of the Labour 70s cabinet, his insane quest to nationalise very failing company (which still failed) and his bizarre ideas about Britain essentially withdrawing from the world really put me off. Great orator though, and clearly a man of principle. My hero (ish) from that time now is Michael Foot, who did his best to save a sinking ship (and failed) through negotiation and compromise. He too was appalled by Benn.
Britain joining to bombing is purely PR/ propaganda, may increase the risk to us but if you have the power to attack evil and choose not to use it, what does it say about you? On Assad, and possibly Putin - the enemy of my enemy is not always my friend, but sometimes pragmatism to deal with the clear and present danger is understandable. We did it with Stalin.
UAE, Saudi Arabia and Bahrain have all run bombing missions against Daesh, not prominently reported here. We would also need them and others to put boots on the ground. The what do we leave behind question is as you say a killer.