His father and his elder brother Henry were killed at the Battle of Evesham in August 1265.

[citation needed] In 1266, Simon and his supporters were now stuck in Kenilworth Castle, which was previously owned by his father.

[citation needed] After the surrender at Kenilworth, Simon and his younger brother Guy escaped to France and Italy.

In 1271 they discovered and murdered their cousin Henry of Almain (whom they blamed for the death of their father) at the San Silvestro church in Viterbo, for which they were excommunicated.

Simon died later that year from Toscana virus at Siena, "cursed by God, a wanderer and a fugitive".