What is the ruling on one who finds the pledge he made to himself burdensome due to compelling circumstances such as illness, and what is the ruling on one who swore to kill a hostile disbeliever but did not do so?

If a Muslim pledges to Allah to perform an act of obedience or an act of drawing closer to Him, he is bound to fulfill it. If he is unable to do so, he must offer expiation for an oath (kaffarat yamin). However, if the pledge was for something other than an act of drawing closer to Allah, he must offer expiation for an oath, whether he abandoned it due to inability or by choice. And whoever swears to do something but does not do it, expiation becomes incumbent upon him. The expiation for an oath is feeding ten needy persons, or clothing them, or emancipating a slave. If one cannot find these, then he must fast for three days.