Disrespect shouldn't mean 'wish for death'

Labour backbencher John McDonnell made headlines this week with his 'assassinate Thatcher' jibe. Many brushed it off as a joke, but it happens that he is no longer standing for the Labour leadership.
A joke it may have been, and as reassuring it is to realise that there are to my knowledge no aspiring, would-be murderers in the Labour party, it was a joke made in extremely poor taste.
While I do not agree with Conservative claims that it was merely a way to try to grab votes 'from the left' in his run-up to the leadership campaign - ironically he is now standing down, and John McDonnell is hardly known for 'pandering to the party' anyway - I do agree on claims that it was insensitive and just downright distasteful. Humour only works when supported by at least small undercurrents of truth, and so I think to even allude to killing anyone - no matter how much you detest their views or the way they are as a person - is extremely unsavoury and by no means a laughing matter.
The ruptuous applause and laughter that followed his comment, however, spoke volumes, and was perhaps even more worrying. What is it in British society that makes us think it's OK to joke about the loss of life? Especially when being pragmatic, taking into account that Margaret Thatcher is now a very frail old woman. Was this really an appropriate comment to make?
Unfortunately it seems that jokes about 'killing off' those we don't like is just commonplace nowadays in British society, and it's one of the many things that I find repulsive about this nation. Only a few weeks ago, I was at a political gathering and overheard jokes - well, I presume they were jokes - but they were certainly said with extreme seriousness - about wanting to shoot David Cameron. I didn't manage to hide my shock and disgust; it was a natural reflex reaction. But as I sat aghast, I couldn't help but witness the smiles and the smirks that surrounded me. I could barely believe everyone was just letting it go as a casual comment, but then I remembered what country we're in, and England's horrible 'laughing stock' culture where people are targeted to such extremes that jokes often lead to talk and speculation about issues completely arbitrary and irrelevant to the issue, such as their 'ugliness', their warped hair style, and casual chinwagging about the most appropriate and interesting way to have them slaughtered.
Perhaps I'm just oversensitive, but I don't think so. As much as you dislike a person, there is a line you can cross. In my mind it is important to always keep perspective and remember to say the things you really mean.