In Isaiah 45:7, the KJV translates the Hebrew word "rah" as "evil". More modern English translations often opt for the word "calamity" instead. Either one of these translations is viable and could be the intended meaning of the passage.
However, that God is ultimately the Uncaused First Cause of all sinful actions is clear from both scripture and reason, so even if the verse cited does not make the point, the general concept behind the question remains.
So why should we worship a God who has caused our sin?
Let's look a little closer at the question and what it means exactly.
What does it mean to say that someone "should" do something? "Should" according to whom?
In the Christian worldview, God is the one who determines what "should" be done and what "should not" be done. Any other determinations of what "should" be done are arbitrary and subjective, without any universal, objective or authoritative merit. Further, if they are contrary to what God has determined "should" be done, then they are objectively wrong or incorrect.
In other words, if Christianity is true (and it is) then we "should" worship God simply because He says we should (Matthew 4:10; Luke 10:27; Acts 17:30; Revelation 14:7).
It's important to remember that in the Christian worldview, it is incoherent to suggest that God can be wrong, mistaken, or in error. The concept of "wrong" is meaningless when applied to God, as He is the one in ontological authority. To be "wrong" is simply to be contrary to the ontological authority. Thus, if He says we "should" worship Him, then we should worship Him. From a Christian perspective, it is ontologically (inherently, objectively, necessarily, logically, naturally, essentially) incorrect to suggest the contrary.
God's Word is true. Always. The contrary is impossible.
Therefore, we should worship Him because He says so.