Though it is simple to say that you should always do right and never do wrong, it is not a simple thing to do. Even taking into account that we cannot achieve this goal due to our fallen nature, there sometimes seems to be ambiguity about what is right and wrong.
As a scientist this is a problem for me especially in the area of animal ethics. It is an unfortunate reality of most neuroscience research that it requires animal experimentation. What is acceptable and what is not is often ambiguous, though despite how bad I may feel about what I do, I know that I am right with God.
God created us in his image, and part of this image is to rule the Earth (Genesis 1:27-28). The practical outcome of this dominion depends strongly on interpretation. I believe that we do not have the right to do whatever we wish whenever we wish, but that we should show respect to all things that God has created, because all that God has created is good.
But God clearly differentiates the value of a human from other animals. The practice of sacrifices under the old covenant demonstrates this: animal sacrifices were only temporary sacrifices for our lives that should have duly been lost. The extension of this can be seen in the cross; only a man (Jesus) can be a suitable sacrifice for the life of a man (us all), no number of animals can amount to the worth of a single person (Heb 10:4). This does not mean that animals have no worth, but God values us so much more.
Matthew 6:26, "Look at the birds of the air; they do not sow or reap or store away in barns, and yet your heavenly Father feeds them. Are you not much more valuable than they?"
Humans are worth infinitely more than animals, and our treatment of animals should reflect this. As such, the practice of speciesism, which is the practice or belief that humans have more value than non-humans is valid.
Yet our treatment should reflect the value of all God's creation. It is in these two considerations that all ethics in regards to animal welfare rest. It is about the weighing of benefits to humans and respecting God's creation. In this way when I consider experimentation I try to find the ways in which to do the least harm to God's creatures and still benefit humans.
This includes minimising pain and the numbers of animals used. In fact any animal experiments performed in universities require these two factors to be carefully considered, and are reviewed by ethics committees ensuring maximal benefit for minimum harm.
However, as a Christian it does not end there. All of creation, which God has deemed as good, must be tended. This includes not being wasteful, polluting, careless or destructive. Even how much and what we eat should be considered. We were given dominion over the earth not to do as we please, but to tend to God's creation, "The LORD God took the man and put him in the Garden of Eden to work it and take care of it" (Genesis 2:15).
So all we do and think should keep in mind the value of human existence (infinite), and minimise the damage done to animals and the environment, which whilst finite is still very valuable. In this way I feel justified with God; my work will benefit humanity but also respect the value of God's work. Just because human life is of infinite worth and value to God it does not mean everything else is worthless.