an adverb which shows that s1 was the intended goal, but instead s2 happens. In combination with a Conditional forms (〜ば、〜たら、なら、と) or some 'or' adjacent word like けど・が.
お母さんを元気にさせるつもりだったけど、却って怒らせてしまった。 I had the intention of making mum happy, but instead it happened that I made her angry.
The actual intended goal can be only implied (not explicit) too.
僕がいると、かえって邪魔になると思うんです。 if I am here, I think that instead it will happen that I am in the way.
お手伝いのつもりでしょうが、却って迷惑です。 I intended to help, but on the contrary I ended up being a nuisance.