I want a scythe to be used for a mass murder, far-range combat, & close-range combat, and I need it to be large to create fear in opponents. Although the massive scythes shown in anime can be destructive, I think they're hard to use in tight places and hallways. I thought about making both sides of the blade sharp but the impracticality of the distance between the handle and the blade itself is still unsolved. I need a way in which it's practical but I can still keep the size. The unbalanced weight is not an issue.
As others have said, long range and short range capability isn't very feasible in one multipurpose weapon, but why not carry multiples? Without any context regarding magic, technological advancement or historical accuracy, I decided to stick to a more reality-based answer and included a quick sketch to illustrate my point.
Use a large, impressive scythe as your signature weapon. It's big, it's scary, it cuts through small groups of bunched up enemies like so many blades of grass. But make it foldable (see pic, item C). A two-part handle and the blade folds down in line. Have it fold outward against the direction of swing so it won't do any inconvenient fold-up when you slice, plus you get to pull off cinematic all-in-one unfold and slice manoeuvres. Strap to back when not in use.
For tight corridors, pack a couple of sickle-sized single handed scythes (picture, A and B) to weave through enemies at close range and for quiet assassinations. If weighted correctly, these can be thrown like tomahawks to kill at range, or for a less practical but impressive looking weapon, connected with a quick locking mechanism to make a large whirling shuriken of death.
The scythe can be moved in 4 dimensions.
If you can move the scythe kata and ana, you can move it entirely into an adjacent 3D plane and out of ours. You can move as much of it back into our 3d plane as you see fit.
For close quarters you might just have 5 inches of the blade, apparently protruding from your palm. Or you can shift the whole thing into plane, swing it, shift it back out of plane to miss an obstacle, and back into plane to intercept your target.
An interesting aspect is what the entirety of the scythe actually looks like. Maybe it is just a bigass scythe. But there may be more to it than that. Its 4d form might be something very different.
How exactly you swing a thing in the kata direction is left as an exercise for the writer. I am sure it takes practice.
There is no middle-aged weapon that is going to be good in all situations. If you want far-range combat, you sacrifice closer range combat and if you want close range combat you sacrifice far range combat.
The entire concept of a farming scythe used as a weapon is flawed. As you mentioned yourself, its a large weapon with an extremely small attack range. Its basically saying, I can kill you if your close to me and You can kill me if your close to me at the same time. Anime, Mangas and Cartoons greatly exaggerate the effect of any weapon because they aren't about the plausibility, but more into the coolness factor.
If you do want to use it for mass murder, I would suggest not using a single physical scythe, but throwing the scythe or shooting it. You get your long range. For close range, just pull out another scythe or have the thrown scythe defy physics and return to you after cutting 10 people apart and hitting a wall.
I suggest you make the scythe blade s-shaped, with the handle attache to the center of the 's' curve. Sharpen the inside curve on the inside, and sharpen the outside curve on the outside. The inside curve is for close contact, and the outside blade is for more distant combat.
Mass killings? Sounds like you either need a big scythe or tons of smaller ones. For the manipulation of many small scythes you could have some sort of telekenesis army of Psions. They could even control tiny scythes which they can conjour in the brains of their enemys to quickly rupture key brain components.
Going the big route perhaps they used hand scythes as a throwing weapon but then adapted them into a ballistic weapon of massive scale. Sort of like a trebuchet but it creates rotational energy flinging the scythe in a circular path of death.
Why just one? Have the guy be able to hurl large ones with his mind and conjure spirint scythes to extinguish key components of an enemy. Could lead to interesting moves.