What are the motives of these terrorist groups?
Strike fear in the heart of infidels?
Help erode the freedoms that citizens in most Western countries enjoy? (especially those freedoms that enable lifestyles hated so much by Islamic State supporters).
Revenge for whatever wrongs they think were done to them? (regardless of justified or not).
As a show of force? Letting everybody know they're still here, and capable of carrying out coordinated attacks abroad.
Just to name a few - surely there's more. In the end they will not accomplish their goals. First and foremost because Islamic State seems incapable of leaving peace-loving 'neighbours' alone. Action = reaction, a law of nature. If they'd just pick some limited area to call theirs, focus on primarily peaceful (r)evolution, and leave their neighbours in peace, they might have a chance at getting just that. But if they want to conquer the world through violence, and stop at nothing in the process... well then: nope, not gonna happen.
Second, because declared enemies of Islamic State aren't going to give up their lifestyles, countries, democracies etc NO MATTER WHAT. As a typical Westerner, personally I could care less what they do in Syria, Irak or neighbouring countries. But any Islamic State fighter is welcome to try and pry my freedoms from my dead fingers in a fight on my turf. Likewise, when their actions cause millions of refugees to "invade" our countries, how could they possibly expect us to not care? Again: action = reaction.
Lastly, because no matter how many crazies are out there, they are vastly outnumbered by regular / reasonable / peace-loving people. And quite a few of those have guns too. And planes. And bombs. And an intelligence apparatus. It's like a car picking a fight with a freight train... yes it'll be ugly, but the outcome is certain: a car cannot possibly win that fight.
What does killing people randomly accomplish?
In the context of these Paris attacks, you may want to re-think in how far victims were chosen "randomly". Not to suggest in any way that victims were known or specially selected. But hey if you start shooting on a busy Paris street, you're pretty likely to hit French people, right?