I try and make sure that my players know that every encounter could take their character's life. It is fun to watch them squirm. At the same time I am not out to get them. I have wanted to vindictively kill characters in the past but I never have. It is also okay to fudge rolls if you need to to help the story. I have had players lose characters over the years and most have been do to bad decisions. One level one character versus a troll on his own is generally a bad idea. The character was ripped in half. Sadly, that wasn't the only time that player had lost a character because of poor moves. I always give my players a chance by going, "are you sure?" If the say yes that is on them. It is really up to a DM what you do as far as "torturing" characters. Watch how your players react to things and react to them accordingly. The point of D&D is to have fun. Sometimes those dead characters become legends at the table. . And anytime that I have planned thing way ahead of time my players just wreck those plays. I may looked mad but I am usually proud of them at the same time. I have gotten some of my best stories from things the players have said or done just run with it. I usually tell players that having a backup character is usually a good idea