As per my knowledge, according to the concepts of Karma if you be bad (sinful) in your life you'll be reincarnated or if you be good you'll be going to heaven. So bad men will undergo rebirth, then who's going to hell?
No. If one does bad deeds first one goes to hell only (provided he did not atone for the sins while living). One does not get rebirth immediately.
And, after the torments of hell are over, one gets born again as various beings, depending on the residue of the sins the soul is still having.
Manu Smriti 12.54. Those who committed mortal sins (Mahapataka), having passed during large numbers of years through dreadful hells, obtain, after the expiration of (that term of punishment), the following births
12.55. The slayer of a Brahmana enters the womb of a dog, a pig, an ass, a camel, a cow, a goat, a sheep, a deer, a bird, a Kandala, and a Pukkasa.
Similarly, from the Vishnu Smriti's chapter XLIV:
- Now after having suffered the torments inflicted in the hells, the evil-doers pass into animal bodies.
- Criminals in the highest degree enter the bodies of all plants successively.
- Mortal sinners enter the bodies of worms or insects.
- Minor offenders enter the bodies of birds.
- Criminals in the fourth degree enter the bodies of aquatic animals.
- Those who have committed a crime affecting the loss of caste, enter the bodies of amphibious animals.
- Those who have committed a crime degrading to a mixed caste, enter the bodies of deer.
- Those who have committed a crime rendering them unworthy to receive alms, enter the bodies of cattle.
Also, in heavens, the stay is not permanent. After merits (Punya) expire, they will throw one out of the heavens :D and one will be back to where he was.
So, to answer your question
Who's going to hell?
It's the sinners who go there, provided they failed to perform the necessary Prayaschittas (or the expiation measures) while living.
Manu Smriti 11.52. Thus in consequence of a remnant of (the guilt of former) crimes, are born idiots, dumb, blind, deaf, and deformed men, who are (all) despised by the virtuous.
Manu Smriti 11.53. Penances, therefore, must always be performed for the sake of purification, because those whose sins have not been expiated are born (again) with disgraceful marks.
So, the pattern is this:
Good deeds-----> Heavens-------> Rebirth
Bad Deeds------> Hells----------> Rebirth.
And not like as you have said. (viz: good deeds-heavens and bad deeds-rebirth.)
Every being has all 3 qualities in more or less forms within them:
- sattva - illumination
- rajas - passion
- tamas - ignorance
Those with increased quality of tamas / ignorance or darkness, go to hell.
While rajas holds at whichever region one is; And sattva promotes to higher regions than the present one.
BG 14.18 - Those with sattva (goodness) go higher [regions]; those with rajas (passion) remain in the middle [regions]; those with tamas (ignorance), who conform to the lowest actions, go down [regions].
Some general context is mentioned in this answer.