Now what was the Karmic reason that Bhishma Pitamah had to Suffer on a Bed of Arrows? To the question as to why Bhishma Pitamah was suffering this punishment even when he had not committed any sin during past 72 birth lives (Great Bhishma could peep into his past lives through Samadhi), Lord Krishna replied him that he committed a folly in past 73rd life birth when he inflicted injuries to insects by piercing thorns / needles into their bodies. Those thorns have now appeared as bed of arrows for you. Your sinful Karma remained dormant as you were a pious person throughout theses lives but since you have now become a party to Adharma of Duryodhana, the sanchit evil Karma’s of 73rd life has ripened now.
So if a person has done good Karmas in past lives, he/she gets a good life in future births & if he/she has done bad karmas in past lives then that person has to suffer bad fruit of those bad Karmas. 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 (after doing good Karmas) , 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 (i.e. as a man or woman on earth); and when one dies in the mode of ignorance (after doing bad Karmas) , he takes birth in the bird & animal kingdom."
So it is said that we should do all our Karmas for the God only as by doing such Karmas no evil result shall come out of those Karmas in future lives.
"yajnarthat karmano ’nyatra
loko ’yam karma-bandhanah
tad-artham karma kaunteya
mukta-sangah samachara" (Bhagwat Gita: Chapter Three verse 9)
"Sri Krishna said: Work done as a sacrifice for Vishnu has to be performed, otherwise work causes bondage in this material world. Therefore, O Arjuna, perform your prescribed duties for His satisfaction, and in that way you will always remain free from bondage."
So let’s do Good Karmas only so that we shall get Goof Fruits of those Karmas in future births. Let’s do all the Karmas for God only as by that way God shall protect us always.