No the cirlce doesn't open portals. If the entity you're trying to summon answer you summon, they'll be there in a matter of seconds. They can hurt you, yes. Killing is a bit more complicated...
If you are stuck with a powerful race of entitie and they do not get away with anything, they may try to lead you to suicide, so you'll kill yourself. They may drive you crazy. It depends on the entity. Entities who want to kill like few Demons, don't usually have the ability to do that staight away, for example heart attack. So they may try to drive you crazy and kill yourself. Other entities such as Gods are much more powerful, I believe they can kill you in their ways.