The good news is that, according to the sagulator, your design should be able to hold a static load of 120kg just fine, and the combination of butt joints and angle bracket reinforcements should support that unmoving, static load as long as you use lag screws or heavy-duty construction or wood screws. Do NOT use drywall screws; they are too brittle and are more likely to shear or break.
Now for the bad news. Your structural design is rather weak, and with 120kg moving around a little on top, I would be very concerned about the whole thing starting to lean toward one side (racking) before catastrophically collapsing. I'll cover a few improvements you can make to make your design stronger.
Screws into endgrain do not hold very well
That section title is pretty self-explanatory. If you drive a screw through one board and into the endgrain of another board, the screw will be much more likely to pull out of the endgrain than it will if you drive the screw into the face or edge of the second board. Toenailing or its fancy cousin, pocket hole joinery, takes advantage of this concept and basically runs the screws in the opposite direction (but at a slight angle, since that's more practical than growing a tree so it will have screws sticking out of its end grain).
Reinforce against racking
Simply put, racking is any lateral motion where the top and bottom of a structure are moving in opposite directions relative to one another. If you don't properly reinforce against it, racking force can exert tremendous force on your joints, especially right-angle joints, causing the joints to fail.
I would highly recommend adding some diagonal braces across the back and sides to reinforce against racking. Or you could install plywood or MDF panels across the sides and back to make the structure more rigid. You could miter the ends and add them in place of the small angle brackets. Or you could use a much larger angle bracket that extends out further from the corner. Either way, you want your structure to be made of big triangles or you want to fill in the rectangles to resist racking.
Upgrade your lumber
The boards you've chosen seem a bit undersized for a bed, even though they certainly can support the static 120kg load. Certain dense wood species will be fine, but if you're using common construction lumber like Douglas Fir, I'd scale up to a larger size. I'm not sure what lumber sizes you have available in your region, but 48mm x 68mm is roughly the size of a 2x4 here in the states. I personally would opt for the metric equivalent of a 2x6 (1.5"x5.5") or even a 1x6 (0.75"x5.5") over a 48mm x 68mm, i.e., a 2x4 (1.5"x3.5").
In addition to the greater load-bearing capacity, you can spread out your screws a little more, which will help strengthen the bed somewhat against racking forces.
How are you going to get it in and out of the room?
You probably don't want to build a complex structure like your design entirely in the room, and even if you did, you aren't going to want to completely dismantle it in order to get it out later. If you examine mass-produced wooden frame beds, you'll notice that the bed often comes apart very easily by lifting and removing the sides, leaving you with two sides, a headboard, and a footboard. Redesign your loft bed so it's made up of a few components that can be easily carried through a zig-zag hallway, up twisting staircases, and through a person-sized door. Consider using knock-down furniture connectors such as the bed rail brackets I mentioned in another furniture design answer.
Upgrade the butt joints
You can replace the butt joints with doweled or mortise-and-tenon joints, pocket hole joints, half-laps, or any number of other joints or combinations of joints. Pretty much anything is stronger than a simple butt joint, though with some of the other reinforcements I've mentioned above, the butt joints will be strong enough.
That said, you can add cleats on the inside corners of your butt joints so that you can screw into the edge or face grain of both pieces, rather than relying on a screw holding in endgrain without splitting the wood.
Also note that these other types of joints either require glue or can be further strengthened by adding glue. Granted, you may need to make special considerations for glue joints if you also want to make the bed knock-down, but it won't be too difficult.
