The first is another reminder of how difficult it is to combine politics with humanity (honesty perhaps). If you'd canvassed Labour activists in 1984 I suspect that a sizeable minority would have agreed with O'Farrell's slightly guilty regret that the bomb had missed (hell, you could get a pretty fair number agreeing that another bomb should be sent across now). To see O'Farrell castigated for it now could be seen as unfairly selective. Moral: if you want to be a politician, learn to self-censor.
But the second lesson is more profound. O'Farrell is (or certainly sees himself as) a nice, decent chap. He was chairman of governors at his local school, he's married with kids, and he positions himself squarely on the cuddly, jumper-wearing British left. In Things Can Only Get Better he writes about how much nicer Labour people are than the braying, posh Tories:
Complex political debate is all very well, but generally most issues can be sorted out simply by deciding who are the nicest people. So that's all that Socialists needed to say: 'Vote Labour because we're nicer'.
Armoured by this niceness, O'Farrell (and this is by no means particular to him) is then free to dismiss his political opponents as wicked, malign people who are actively trying to harm people. For another example of this see Johann Hari, who wrote a matter of weeks after the death of David Cameron's first born son:
If Cameron announced the slaying of the first born, he would be applauded for having a great policy for second children.
It's no great step from here to no longer see your opponents as really being people at all. Possibly the left's greatest hero, after all, famously said that Tories were lower than vermin. Everyone knows that line, but the full quote is even more interesting:
No amount of cajolery, and no attempts at ethical or social seduction, can eradicate from my heart a deep burning hatred for the Tory Party. So far as I am concerned they are lower than vermin.
For a party that has 'niceness' embedded so deeply in its self-image, there's a disturbing amount of hatred on the left. And the way to square the contradiction is to refuse to see your opponents as properly human. That way, you can hate them, wish them dead even, without it meaning that you're not the nice guys. I do wonder though whether O'Farrell would be able to keep this up in person. If Norman Tebbit sat down with John O'Farrell and talked him through how he looks after his wife, paralysed by the IRA bomb in Brighton, would O'Farrell be able to keep up the line that they're nothing more than the enemy?
So it's not really a question of whether Ed Miliband should repudiate John O'Farrell, it's whether a whole section of the left should re-evaluate their core political beliefs - and that's rather more of a stretch.