This malady suffered by many kyu level players is the mistake of wanting to kill just about any group of the opponent's on the board that the player feels they might have the slightest chance of killing.
Dieter: May I point out the disease that struck me by taking an overdose of the above described antidote: failure to kill a killable group, being satisfied with taking points chasing it, and losing on points.
HolIgor: I've noticed that the disease of the kyu players is rather than going for an unprepared kill, instead going for a kill when you are a little bit too thin. This can be disastrous. At the same time Dieter speaks about the situation when it is difficult to say if your were already prepared for a kill. Something where you have to read it out carefully and you couldn't, usually, because of the lack of time.
Charles The Greeks knew about this, a long time ago. Nothing in excess.
Alex Weldon: I guess it essentially comes down to how confident you are in your reading abilities. It's certainly a bad habit to try single-mindedly for a kill when you're not at all certain that you'll succeed (you end up with amarigatachi, more often than not). However, if you read a situation out, and you feel reasonably sure that you can, indeed, kill a largish group of stones (without hurting your position elsewhere too much), surely it's a mistake to pass up the opportunity. If you're like me, you often read things wrong... but it's still probably good to go for the kill, because if you're wrong, you'll probably learn something from the experience.
Charles One of my erstwhile pupils finally beat me in an even game; and afterwards made the comment that on the six previous occasions he'd had a group of mine which he thought was dying escape via ko. Sometimes reading it all out is beyond our ordinary capacities, and I'm sure at that point it is worth making a judgement instead. Landing a punch isn't the only way to win.
Andre Engels: It will often be as Charles describes, that one cannot decide whether killing will work or not. Often the better way of making the decision, is to look what will happen if you do not try to kill the group. If a more lenient attack is likely to end you in a favorable or equal position, going for the kill is a (too) risky way to play. If without killing you would be behind (to the extend that you can count it), then taking risks is exactly what you want to do most.
BlueWyvern: I actually have observered the inverse problem. A lot of kyu players seem to think that all invading stones are invincible and make exceedingly brash invasions. Since I've gotten better at positioning my thickness (and since reading AttackAndKill) something like %60-70 of my games have ended in me killing one, two, possibly even three big groups in the center. Looking back on these, my opponent could have gotten away in most of these cases with a simple reduction. I'd say the Kyu disease is a general disregard for safety in matters of life and death.
Malweth: I think the problem changes as the kyu level changes. (in most cases) The DDK trys to kill anything entering his territory, however unsecure that territory is. The SDK realizes that not everything can be killed and is therefore willing to take the risks of too-deep invasions yet also misses many possible kills. So when does the knowledge of what can and can't be killed come?
Notochord: Not until after 9p, certainly. Learning (and error) is all a relative matter.
chrise I suggest that players try both ways; all out attack on the one hand or careful probing on the other, and find which way suits their style best. There is a lot to learn in trying these things out. Me? I'm now of the careful probing type :)
Thad: I don't want to kill everything. I just want my opponent to sweat making it live.
Cazort: I find that the only games in which I win by killing groups (and I am certainly a mid-kyu player as I write this) are those in which I would win even if I did not kill the group. There are so many useful ideas, make territory while attacking, or lean on one group to attack another, that kyu's understand, and can carry out when they think about them, but that they haven't fully integrated into their gameplay. I find that when I try to kill for the sake of killing, it usually lives, and then I lose. If I try to gain by threat of killing...then if it lives, I get something small, if it dies, I get something big.
Warder05: I (14k) seem to be suffering from a variant form of this disease: lack of balance. I ever since I've started to empahsize sente over correcting defects in my shape (a habit to which I actually attribute an increase in strength), I've found that I occasionally continue to press an advantage when I really should be protecting. I'm thinking that maybe improved positional judgement will solve not only my problem, but many of the problems mentioned on this page. Shouldn't understanding the size of the threat in relation to the current board situation help? I mean, if you don't need to kill something, then there's no reason not to just seal the group in.
Uurtamo: Sometimes when one starts to chase something that looks easy to kill, it gets big enough that it becomes very, very hard to kill. This very hard to kill group then acts like a 50 foot high-voltage cable that's been cut free from its mooring, flopping around and killing everything that touches it. This extremely painful experience has occurred to me enough times, and happened so quickly, that I've decided to commit myself to adapting my play in the following way: if I don't see exactly which points to play that will remove all eyespace of a group in sente, then I will threaten only in order to build or save my own skin, and never actually commit stones to kill. Unless, of course, there's nowhere for the group to run, in which case I will only take gote to kill it if I have nothing larger to save. Now if only I could differentiate between unreasonable invasions and sente reduction... eh, let's just be honest, I have kyu disease.
Tapir: Honestly, I believe the opposite is quite often true as well. Polite play where nobody tries to hurt the opponent at all, every invasion being considered overplay etc.