YUCK! A rat’s nest like this is confusing and dangerous. Cut out the old slings and pack them out, and maybe add a new one of your own to make it simpler and stronger.
Carrying a blade of some sort can be a smart thing to have on alpine climbs. No, you’re probably not going to pull a Joe Simpson (see below) and have your partner slice the rope and drop you to free themselves; the normal cases for knife use aren’t nearly as exciting.
What are some uses for a knife when climbing?
Cleaning up ancient, sun baked webbing from rappel anchors, and cutting some of your own new webbing or cord to enhance existing anchors
Some sort of self rescue situation, when some piece of rope or webbing is loaded and it needs to get unloaded, and the only way to do that is to cut it loose
Maybe your rope gets damaged and you want to cut away the bad part
and, maybe prepare the occasional peanut butter sandwich
There are three general approaches to climbing knives: razor blade, tiny cheap lockblade, or nice designated climbing knife.
If you want to carry a blade for strictly emergency use, it’s hard to get much more lightweight than a single edge razor. Put a few layers of athletic tape over the blade to cover it. Some people tape the razor inside their helmet, or keep it in a zipper pocket on the chalk bag. Are you in the fast’n’light camp and want something for emergencies only? This is a good choice.
Cheap tiny lock blade
This is my personal pick. I got one for under $5 next to the hardware store cash register. It’s tiny, has a wicked sharp serrated edge, and it’s a lock blade. The serrated edge can be especially helpful for cutting webbing. Be sure there is a hole in one end of the knife so you can add a little keeper loop to clip to a carabiner. Make the loop with a bit of of bank line or 2 mm cord. If you find yourself doing more alpine routes, where your liable to find funky anchors, this could be a good call.
(I keep this knife on a small locking carabiner, along with a Ropeman mini ascender and a small pulley, and call it my “Oh S**t Kit”.
Whatever knife you choose, it needs to be absolutely secure so the blade can never open accidentally, to cut you or your equipment. You can tape it securely closed with athletic tape, and or add a thick rubber band cut from a discarded bicycle inner tube, or both. Below, I’ve added a stout rubber band from an old bike tube to secure the blade.
Carry a “real” knife
A definite step up in quality from the hardware store lockblade is this nice little knife, the Trango Pirana. It folds, and has a clever design that prevents it from opening if a carabiner is clipped through the large hole.
If you think you’ll be using your knife for more day-to-day things, such as slicing bagels or making that peanut butter sandwich, you’ll want something with a longer blade. Petzl makes a popular model called the Spatha. Personally, I don’t feel the need to carry a blade this big for occasional use, so this is not my choice, but it could be an option for you. (Note the large hole that can take a carabiner.)
If you really have to cut the rope in an emergency situation with no knife, I‘ve read you can do so by taking the thinnest cord you have, and sawing it rapidly back-and-forth with your hands through whatever needs to get cut, and theoretically the rope will eventually get cut by friction. I’ve never tried this, but it might be something to put in the back of the toolbox if you ever need to do this.
One of the most harrowing climbing epics of all time was endured by Joe Simpson in his classic book, “Touching the Void”. On a challenging climb in Peru, Simpson was being lowered by his partner down a steep face, was lowered into a crevasse, and his partner above was stuck. He made the agonizing choice to take out his knife, cut the rope, and drop Joe to what he thought was a sure death. A classic read it if you haven’t.