To put it another way, is it because the expectation of a reward nullifies the benevolence of the act? If so, how does Christianity address this?
Your quote from Isaiah is similar to a quote from Romans;
Romans 3:10 As it is written, There is none righteous, no, not one:
These attestations of original sin or the inability of humans to raise themselves to righteousness indicate our fallen state. This is usually observed in religious practice as a type of theater;
Matthew 6:2 Therefore when thou doest thine alms, do not sound a
trumpet before thee, as the hypocrites do in the synagogues and in the
streets, that they may have glory of men. Verily I say unto you, They
have their reward.
Christianity is rather unique in worlds religions. Most religions are works based and are described in terms of what you have to do to get something. Even some Christian denominations have been packaged as a "do this, get that" religious system.
At the core of Christianity is the gift of eternal life that is available through trusting in Jesus. There is nothing that has to be done to earn this gift.
True Christianity is about transcendence. Once we have new life in Christ and are filled with his Spirit, we should begin a transition away from those things that appeal to the flesh and are more drawn to those things pertaining to the Spirit. The letter to the Galatians describes much about this.
Christians are supposed to be growing in the image of Christ. This is leaving the world of selfishness and the flesh and maturing in the world of selflessness and love.
True "benevolence" comes from a heart not contaminated by "self". Jesus is our example in this of that to which we should aspire.
Romans 5:8 But God commendeth his love toward us, in that, while we
were yet sinners, Christ died for us.