One of the devil's greatest schemes is to convince people he doesn't exist, because then we won't take him seriously -- and that's exactly what he wants us to do.
But the devil does exist! Not only that, but he is a powerful spiritual being who has great authority over human society. He isn't as powerful as God -- but he still is very powerful, and we must never underestimate his ability to influence people and turn them away from God. And that's his goal: to turn us away from God and His will for our lives. The Bible calls him "Your enemy the devil (who) prowls around like a roaring lion looking for someone to devour" (1 Peter 5:8).
Satan isn't only strong, however -- he's also absolutely evil. Even when his temptations appear to be good, they never are -- because his goal is to harm us and keep us from the good things God has for us. Was he at work in the murder-suicide you mention? I'm sure he was, either openly or behind the scenes.
But you need to know one other thing about Satan: He is a defeated foe! By His death and resurrection Jesus Christ conquered death and Hell and Satan -- and some day the battle will be over and Christ's victory will be complete. Make sure you're on the winning side, by putting your life into Christ's hands today.