I have never heard of the word neglective. If it exists it is not commonly used today. I would not use the word contemplative in the context you give. I would use "quiet". I would use contemplative to mean "deep in thought". But this is a literary word that I would not usually using everyday speech. The only time I can remember using this word recently, was to describe a friend who is a nun. She spent many years in a contemplative order: a group of religious women who spend most of their time praying and meditating.

I don’t know if people really use the word “neglective.” Neglectful is what would be said. Contemplative- means to be in deep thought. But in this sentence are you trying to say you stayed out if the argument? You can say you stayed neutral. Meaning you took no side.