I just roasted a small chicken, and after taking it out of the oven after about 1 1/2 hours, a large amount of watery, red fluid leaked from the bird.   Did I somehow mess up cooking the chicken? Is it edible?  The chicken was cooked, alone, in the oven at a temperature of 180 Celsius (356 Fahrenheit). There was nothing else done to the chicken (no brining or anything). 
As another answer points out, the liquid consists of the liquid contents of the chicken's cells. It is mostly water with some proteins dissolved in it.   I think that your chicken was cooked enough. If the liquid is raw, it will look a lot like slightly watered down blood. This is how most people describe it. The fact that you call it "watery red fluid" already sounds to me as if it had the clear, red-tinted appearance of cooked meat juice. Also, 1.5 hours in a hot oven should be more than enough to cook a chicken, even if your oven's thermostat is discalibrated.   My assumption is that the chicken had previously been frozen, you cooked it too long, and/or cut it up too early. All of this can increase the amount of meat juice which flows out of the cooked meat instead of staying inside and making it juicy and tasty. Sadly, it is not as tasty as a properly cooked chicken. But the good news is that you have no safety concerns whatsoever. So you can eat your chicken.   To make it completely clear, because I haven't seen your chicken: If the juice which flows is bright red and cloudy or translucent, it is undercooked. If it is clear/transparent and the red has a warm, slightly brownish hue, it is probably cooked (clear juices are not a 100% indicator of having reached a safe temperature, but raw-looking juices are a 100% indicator that you have to cook longer).  