Everyone seems to be going fairly high tech or require some special requirements of the environment such as low gravity for such a system to work. But there are real-world options.
So you want a slow moving ballistic (= unpowered) projectile weapon with a range of at least 50 meters that can inflict fatal wounds. Let's explore some options.
The Math
I'm using [this tool][1] to calculate distances, angles and timing.
You said that at 20 meters, a human should be able to dodge. Let's say it takes about 1 second to recognize the projectile and move out of the way. Let's also assume that the humanoid aliens are about our size and fire their weapons from about 1.7 meters in height.
If you were to fire straight at the target - so at a 0 degree angle in respect to the ground, to reach a distance of about 20 meters you would need to fire the projectile at 35 meters per second and you would hit the targets ankle at 0.58 seconds after firing.
Through googling, we know that to maximize distance we need to fire at 45 degrees and we can then calculate that the slowest projectile that can hit at 20 meters needs to travel at about 14 meters per second. This takes 2.177 seconds and since the projectile is coming from above, it might hit more than just the ankle.
For 50 meters, we need at least 22 m/s and it takes 3.3 seconds. So just to be safe, let's say the maximum firing velocity is 25 m/s, giving us a maximum range of 65 meters. Since the aliens are out for blood, they always throw at this velocity and adjust the angle for closer targets.
So at 50 meters with a 25 m/s projectile the aliens can fire at an angle of 24 degrees and it takes 2.23 seconds to reach the target. At 20 meters distance the alien can fire at 5 degrees with a flight time of 0.85 seconds. 0.85 seconds might not be enough to reliably dodge, but someone with fast reflexes can probably still do it.
Regular arrows at about 70 or more m/s are too fast, hitting something at 20 meters away in 0.25 seconds. 347
A baseball thrown by hand by a professional player can travel at 40 m/s giving us 0.58 seconds to dodge at 20 meters (incidentally, the batter is standing even closer and can react well enough to hit the ball with his bat. That's a good sign that your humans can dodge within 1 second).
A baseball isn't very deadly, but it shows that the speeds necessary can be achieved even without having assisting tools. For example, a throwing knife, while not being the deadliest weapon, already fulfills your requirements, although they loses effectiveness at range.
Deadliness
To make a projectile deadly you need force, if there's enough force it will even penetrate the target. A bullet gets its force from its speed, a thrown rock gets its force from it's mass. Something like an arrow is neither very fast (at 70 m/s) nor massive, but by using a pointy arrow head, most of the force can be focused on the very tip of the arrowhead which can then penetrate the target. If you use something like a blunt arrowhead, it won't be able to do much damage.
Since you don't want speed, as long as it's more than 25 m/s, we have to focus on mass and being pointy.
A Javelin, or throwing spear, is a weapon that fits your demands perfectly. The current world record in javelin throw is 98 meters. Using the previously calculated formulas, we can guess that this was thrown at about 31 m/s. These javelins are optimized for throwing distance, weight 800 grams and are only pointy enough to stick in the ground on landing. Even unoptimized, they occasionally hit one of the judges (as searchable on youtube), and occasionally penetrate and stick in them - deaths happen. Incidentally, apparently it's considered save enough for judges to stand on the field - I assume a testament to their dodgeability.
If they're made more pointy and their weight is increased, they should become both more deadly and even slower, so they better fit in your dodgeable at 20 meter criterion again. 347