I have no idea what mud or blood is on his hands, but it seems to me that prosecuting him would certainly discourage other Libyan political figures from defecting ... surely not what would be most helpful in the circumstances.
I suppose this is a very tricky one for the authorities.
Well innocent until proved guilty, but the mud around Moussa Koussa is prety thick and shot through with blood.
I don't think we expect defectors from the Libyan regime to come to the UK - nor do we want them to. It is perfectly possible for Cameron and Hague to say (as they more or less have) that senior members should defect now while they have the chance and go into exile because we will prosecute if we catch them. Equatorial Guinea has been suggested as a home away from home. For that matter I don't think our UK politicians can do otherwise than prosecute. The UK has separate legal systems in England and Scotland, both of which are independent of political control and not likely to consider no-prosecution deals as legally valid. In the end the PM cannot bind the courts - he is not head of state.
All this really does raise the issue of just what Moussa Koussa is up to. One line of thought is that he knows he will be prosecuted and wants to use a legal process as some sort of revenge, possibly on Gaddafi, but also possibly on the UK. Another is that he is offering himself as a new president of Libya to preside over the reforms required (and as a head of state he would have immunity from UK prosecution). This supposes that with coalition backing he could be inserted into Tripoli and pull off a coup which he couldn't pull off while he was in Tripoli - seems unlikely. There's also the idea that he has undergone some sort of crisis of conscience or nervous breakdown and is seeking some sort of spiritual salvation by delivering himself to his enemy (which matches the Tripoli line that he is sick).
Moussa Koussa may well be as dangerous to the UK as to Libya. If he does have the goods on Blair and Brown (as some of the Twitter traffic thinks) then yes he may well be able to get these two into UK courts facing charges, but it is hard to see that the mud would stay with just them. In effect he may be a Trojan Horse. To garble Vergil, we should fear the Libyans even when they are sending us gifts!