You can’t make a deal with the dead
You cannot negotiate with dead men. MI6 and, eventually, the British government recognised that a political struggle requires a political solution. However brutal the IRA’s day-to-day terrorism, a strong, coherent republican leadership was in the strategic interest of the British state.
That fundamental insight still appears to be lacking in the Middle East conflict. If a peace process is serious, each side must accept the other as they find it rather than remould their enemies into a more compliant state by assassination and political diktat.