We recently sustained an aggressive light Sussex cockerel for several months. I never quite believed my husband about how aggressive the cockerel had become as I didn't see it. However, after several passive garden visits to hang the washing out, 'killer' (as my husband called him!) began to attack my shins. On one occasion I thought 'I'm not giving in to you', and I pointed my wellie in his direction for around 2 minutes, after which 'killer' was exhausted. It became apparent that the poor cockerel's life purpose was to protect his brood (although there was no real threat), and he just couldn't operate at a neutral level.
Poor 'killer' had to go, despite having been incubated from egg and hand-reared. This was the first cockerel in 150 chickens who had behaved in this way.