Questions about the 2009 horror film 'The Collector'?

This evening I watched the 2009 horror film 'The Collector. Whilst I enjoyed the gore in it, there are several aspects of the plot that I found confusing.

Firstly, what was the deal with the red box? Why was the man in it, why was it there at the beginning, and why was Arkin put into it at the end?

Second, what was the fizzing orange/yellow stuff on the floor with the cat? Was it acid?

Thirdly, do we ever find out why the collector is doing all these killings?

I have looked on Wikipedia and IMDB but cannot find any info on there to shed light on the situation.