Yes it can be negative.
this condition of diminishing marginal utility is known as law of diminishing marginal utility. eg- on consuming 1 ice cream satisfaction derived will be 5 utils . on consuming 2nd satisfaction will be 6 utils. on 3rd icecream 4 utils . on consuming 4th icecream 3utils. on consuming 5th icecream 2 utils. on consuming 6th icecream 1utils . At this stage the person's limit come to what extent it can consume a commodity. but on consuming one more it can go to -1 as he will not derive the satisfaction but will vomit the icecream out