1) Assassination is a very risky strategy for NATO. Given the number of potential replacements for Muammar Gaddafi within his own family, its unclear that assassination, even if successful, would be a good strategy. Also, a missed attack could generate serious reprisals in Misrata and elsewhere in Libya. Finally, the US policy that "Gaddafi has got to go" has drawn domestic criticism. No doubt this criticism would multiply if the US added assassination to its toolbox of options in Libya. News of the strike may very well have a political cost for the leaders of NATO countries.
2) Yesterday would have been a more ideal time for the strike. The British royal wedding would have overshadowed any news of targeting one of Gaddafi's sons. It also would have given the NATO governments the full weekend to let the story calm down a bit before Monday morning quarterbacking on the news. That being said, it's 100% plausible that the intelligence, capabilities, and decision didn't line up until today. If you think that's the case you can ignore this paragraph.
3) The norm of not assassinating leaders is still very much salient in international politics. Given the expertise with which the Obama administration navigated the norms of multi-lateral intervention with regards to the NATO no-fly zone resolution, it is confusing that they would be so eager to overturn the norm of not assassinating leaders. Gaddafi also escaped the assassination according the the Libyan government, but four others did not.
The intention of raising these questions is not to stir the pot of conspiracy theories vis-a-vis some elaborate scheme Gaddafi has concocted. Rather, it is to point out that the full story behind this strike is still very much unclear. In the 24/7 news age, it is important to carefully consider reports and wait until all the facts are clear before drawing conclusions.
I'll follow up on this post once things become a bit more clear. It's entirely plausible that NATO is in fact interpreting the "Responsibility to Protect" clause very liberally, but it's impossible to know for sure at this point.