あるまい=ある+まい means no, impossible.
1. [Conjecture of the negative result] [はずがない] can't...maybe; […でないかもしれない] maybe not...; [おおかた…でないだろう] probably not...
- たぶんそうではあるまい／Probably not like that.
- まだ雨は降るまい／It won't rain yet.
2. [Deduction of Negative Will] I don't intend to..., don't want to...
- 二度と行くまい／Never want to go again.
- うるさく言うまい／No more details.
So, you find that it actually has a sense of speculation in it.
It's not a direct negative tone.