If you arn't Swedish you probably havn't heard of it just like I havn't heard of similar accidents in other countries. If you are Swedish you probably havn't thought of it much yesterday or today. You would know it if someone asked you but you havn't thought of it. That kind of accidents occur all the time, it is of course tragic but they happen everywhere in the world. There is a reason they are called TERRORists as in terror and not murderers, even if they are of course both. They thrieve on fear which throws suspicion on innocents. Just imagine these innocents being looked at with suspicion and being mistrusted or even seen as terrorists all their lives, won't have good consequences for anyone.
Be terrified but see it how it is and don't overreact. You can also check out the hashtag openstockholm, quite impressive how much people help each other when needed.