Why is Utterson concerned for Dr. Jekyll after reading his will?

Mr. Utterson is concerned after reading Dr. Jekyll's will because his friend is leaving everything to Mr. Hyde, a man Mr. Utterson has never heard of or met. This in itself is strange, because Mr. Utterson and Dr. Jekyll go back a long way and come from the same set of friends. Utterson also worries because the will stipulates that should Dr. Jekyll disappear for more than three months, Mr. Hyde should:

. . . step into the said Henry Jekyll’s shoes without further delay and free from any burthen or obligation beyond the payment of a few small sums to the members of the doctor’s household.

Mr. Utterson's initial thinking, which is that this will is based on a whim or impulse, means it offends his sane and orderly mind. He then starts to suspect that the will may be due to Dr. Jekyll having to hide some "disgrace." Whatever the case, the air of mystery about Mr. Hyde worries him.
Mr. Utterson therefore searches out Mr. Hyde. When he meets him, he is even more concerned because the man is odd, unpleasant, and Satanic. He can't understand why Dr. Jekyll would make such a person his heir. But when he presses his friend about the will, Dr. Jekyll insists that it is a complicated situation and that the will must stay as it is written. Mr. Utterson simply cannot understand any of this.