In Hindu scriptures Karma is related to the origin of one's own body. Karmas of earlier birth become the basis of this birth & Karmas of this birth become the basis of your next birth & so on. At death, soul of a person takes the stock of all Karmas (having its reservoir in that person's mind) & Divinity or God as per the divine law takes a handful of those Karmas & a new body is made with the seeds of these Karmas. So if a person has done good Karmas he/she gets a good godly body in heavens & if he/she has done bad karmas in this life then that person's soul goes to lower wombs of birds & animals. So it is generally said that we should do good Karmas only.
Below are few verses of Bhagwat Gita which explain about how a soul travels from one body to the other & where it goes as per the Karmic law:
"yada sattve pravrddhe tu
pralayam yati deha-bhrt
amalan pratipadyate" (Bhagwat Gita: Chapter Fourteen verse 14)
"Sri Krishna said: O Arjuna, When one dies in the mode of goodness, he attains to the pure higher planets of the great sages."
"rajasi pralayam gatva
tatha pralinas tamasi
mudha-yonisu jayate" (Bhagwat Gita: Chapter Fourteen verse 15)
"Sri Krishna said: O Arjuna, When one dies in the mode of passion, he takes birth among those engaged in fruit producing activities; and when one dies in the mode of ignorance, he takes birth in the animal kingdom."
"urdhvam gacchanti sattva-stha
madhye tisthanti rajasah
adho gacchanti tamasah" (Bhagwat Gita: Chapter Fourteen verse 18)
"Sri Krishna said: O Arjuna, Those situated in the mode of goodness gradually go upward to the higher planets; those in the mode of passion live on the earthly planets; and those in the abominable mode of ignorance go down to the Lower hell worlds."
In above verses Lord Krishna explains the result of “Rajo Gun” or mode of passion being predominant at the time of death is that the jiva or embodied being is born into families that are are involved in vigorous activity. Likewise the result of “Tamo Gun” or mode of ignorance being predominant at the moment of death then the jiva is born in the wombs of irrational species as animals.
Lord Krishna describes that when a jiva or embodied being dies while in “Rajo Gun” or the mode of passion, one takes birth among those who perform activities out of desire for rewards and having been born as such will endeavour mightily to accomplish works that would be rewarded by immense wealth and power even up unto the heavenly spheres. Similarly if one dies while under the influence of “Tamo Gun”or the mode of ignorance then such jivas will be born from the wombs of animals such as pigs and dogs where one would be completely incapacitated to perform any activity that would give any spiritual benefit.
Lord Krishna states that if death comes upon a jiva or embodied being while predisposed to “Rajo Gun” or the mode of passion then one takes birth next from those who are obsessed to perform actions in anticipation of rewards. Also a jiva dying while under the influence of “Tamo Gun” or mode of ignorance is forced to take birth as an animal in the next life.
So we should do Good Karmas only as result of doing good karmas is good only. Those people who do Bad Karmas, do harm to other people & do sinful activities, such people shall have bad consequences as they are in the “Tamo Gun” state at the time of their death. Such “Tamo Guni” people shall take birth in the animal species as a dog, or pig or other animals, or in the birds species or even in lower species as an insect or worm. So Lets do only Good Karmas only in our life.
Jai Sri Krishna