Create a new object monster LR - set it to the blue ghost "monster 1" (not "monster_nice"; we'll use those later for facing directions).
In your testing room, delete everything in your room that we've already tested. Add some walls down the middle with a hole, and put your person there. Add some horizontal walls on both sides as shown below. Then add 2 monster LRs in your room.
Test it out - do they move left & right, and reverse direction when they hit a wall?
The easiest thing is to duplicate your monsterLR object....
Change the name to monster UD, change it to the green "monster 2" sprite.
Add 2 monster UDs to your room as shown below.
Test it out.
Go into your person object and the following:
Duplicate this event for the Collision with monsterUD... (it's the same, you don't have to change anything).
Your person should now look like this...
Test your game - if you run into a monster, do you "die" (pause/sound, person where started)? If yes, continue.