This may be an old issue, but doesn't Paul also say that if someone is offended he would 'never again eat meat'? Also, I wonder how you could write that God would consider killing animals lightly because that is not the impression I get from Isaiah.
I always wonder about people who make unsupported charges. I searched this web site; there is no statement that "God considers killing animals lightly." It might be what you think I believe based on your readings. If so, I would not be surprised since so many people only see what they want to see.
Pulling a small phrase while ignoring the context in which it was offered could give the impression of something being stated that the author did not intend. Politicians do it all the time to bring down their opponents. You quote a small phrase to make it look like Paul has given up the eating of meat -- which never happened. The phrase comes from the conclusion of a conditional:
"But beware lest somehow this liberty of yours become a stumbling block to those who are weak. For if anyone sees you who have knowledge eating in an idol's temple, will not the conscience of him who is weak be emboldened to eat those things offered to idols? And because of your knowledge shall the weak brother perish, for whom Christ died? But when you thus sin against the brethren, and wound their weak conscience, you sin against Christ. Therefore, if food makes my brother stumble, I will never again eat meat, lest I make my brother stumble" (I Corinthians 8:9-13).
In context, the meat Paul is talking about is not all meat, but meat sacrificed to idols. What Paul is stating is his willingness to restrict himself in regards to something that he clearly has a right to in order to not cause another person to sin against his own conscience. Paul is talking about purposely forgoing a personal right to help teach a weaker person. Romans 14 is about the same topic.
"Yet if your brother is grieved because of your food, you are no longer walking in love. Do not destroy with your food the one for whom Christ died. Therefore do not let your good be spoken of as evil; for the kingdom of God is not eating and drinking, but righteousness and peace and joy in the Holy Spirit. For he who serves Christ in these things is acceptable to God and approved by men. Therefore let us pursue the things which make for peace and the things by which one may edify another. Do not destroy the work of God for the sake of food. All things indeed are pure, but it is evil for the man who eats with offense. It is good neither to eat meat nor drink wine nor do anything by which your brother stumbles or is offended or is made weak. Do you have faith? Have it to yourself before God. Happy is he who does not condemn himself in what he approves. But he who doubts is condemned if he eats, because he does not eat from faith; for whatever is not from faith is sin" (Romans 14:15-23).
But a willingness to change personal behavior to keep another person from offending their conscience doesn't change the fundamental fact regarding what is a right given to mankind. Paul also said:
"Now the Spirit expressly says that in latter times some will depart from the faith, giving heed to deceiving spirits and doctrines of demons, speaking lies in hypocrisy, having their own conscience seared with a hot iron, forbidding to marry, and commanding to abstain from foods which God created to be received with thanksgiving by those who believe and know the truth. For every creature of God is good, and nothing is to be refused if it is received with thanksgiving; for it is sanctified by the word of God and prayer" (I Timothy 4:1-5).
Clearly, just being able to pull a few choice quotes from the Scriptures doesn't mean that the Scriptures teach what you desire. In reality, pulling a partial statement to make an author appear to say the opposite of what he believes is a form of lying.