"It is not necessarily bad."
わるい is an い adjective so you drop the い and add くmost of the time when it is not the subject of the sentence (basically when it is at the end of the sentence it is わるい but if you say something becomes bad it is (わるく)に なります
わるい is an い adjective. when it forms the negative い changes to く。making it わるく。
わるく on its own means badly. In this case, it's better to think of わるくない as "not bad" as opposed to "good".
To everyone wondering what the ha means it is a way of partial negation. http://www.guidetojapanese.org/forum/viewtopic.php?id=1686