’Make them pay now for what they’re doing’

Here, again, is the context for the direct quote of Thomas Mulcair’s that appears in the new Conservative attack ad.

What did he mean by it? Well, in the next sentence he refers to the "polluter pay" principle.

As noted here previously, "polluter pay" is a principle to which the Conservatives—or at least Stephen Harper and John Baird—have pledged allegiance in the past.