1) Your loop doesn't go full circle. You can draw another connection from the assignment element 'assign boolean if found match' and the loop element 'LoopAccount'. Because there is no connection back to the loop element it in effect isn't a loop, and the flow ends after the assignment element. (I would suggest using an extra assignment element as a place holder to make a loop an actual loop. Because an assignment element requires a variable, you'd need to create an empty variable just for that.)
2) If you have made sure in advance that every domain only exists once in your database, then you probably don't need to loop over any elements. You can simply put all the accounts in an s-object variable (instead of an s-object collection variable). The fast lookup will than put the first one it finds into the s-object variable. After that you can then use that decision to check if the variable is empty.
I hope that helps you.