On earth, during mortal life - yes, of course love can be bad; and often is.
If the love is illegitimate, then its fruits will be bad, and thus we we know.
Love of God cannot be bad, but love of neighbour can be bad - often, perhaps usually is bad - insofar as it usurps rather than sustains the love of God.
Charles Williams adulterous love of Phyllis Jones was bad, and can readily be seen to be bad by its fruits.
On earth these fruits included immediate and growing dishonesty, then hurt, then destruction of wholeness, then misery - and in ultimate terms this unrepented evil earthly love, led to displacement of Williams' love of God.
From about 1937 C.W focused on the idea of coinherence by means of exchange and substitution; and this increasingly became conceptualized as a non-Christian technology, a magical not religious thing.
From Alice Mary Hadfield's An Introduction to Charles Williams (1959):
Page 138: From  on, the word 'love' has always a double meaning in his thinking, as personal love, and as love in the City which may be felt in any of the concerns and relationships of people everywhere.
[Note: no reference to love of God.]
Page 141: Can [coinherence] be done without belief in Christianity, without belief in God at all? I [AMH] would say not, though C.W clung to his claim to talk on equal terms with agnostics and non-believers. My objection to [C.W's novel] Descent into Hell [of 1937] is that by silence about Christ it is implied that the life of exchange can be lived without knowledge of Him.
Page 133: Just as he was not particularly interested in people's personalities but much more in their ideas and behaviour, so he was not curious about the personality or human details of Jesus and His life at Nazareth in the Gospel story... It was the paradox of the union of God and Man which held C.W's ,ind, as it had Karl Barth and again the modern theologians of the century.
Thus we perceive that fatal flaw in C.W, along with 'modern theologians' which has led to the current anti-Christian heresy of Liberalism - the deadly heresy of forgetting or displacing of love of God, and the consequent and false assertion of the Goodness and indeed primacy of love-as-such, of earthly love, of love of any-thing under any circumstance - of even illegitimate love.