I'm not so sure. They could have just been doing something dodgy but not illegal but Trump decided to keep escalating things rather than back down because that is what he always does.
Its also very possible that they didn't actually break the law or anything in the campaign but they did in the cover up in the subsequent months.
I don't think this was some huge evil plot that Trump and Putin put together either before or at some point during the campaign. Maybe evidence will eventually prove me wrong, but that's my feeling at this point.
If anything, I think Trump likely approached it like he would in the real estate world where you have to create relationships with some shifty characters to get business done.
Of course adding in Flynn, Manafort and Trump's tendency to lie at will has blown this up into the scandal it is.